Anode active material, manufacturing method thereof, and non-aqueous electrolyte secondary battery

ABSTRACT

In order to provide a 3V level non-aqueous electrolyte secondary battery with a flat voltage and excellent cycle life at a high rate with low cost, the present invention provides a positive electrode represented by the formula: Li 2±α [Me] 4 O 8−x , wherein 0≦α&lt;0.4, 0≦x&lt;2, and Me is a transition metal containing Mn and at least one selected from the group consisting of Ni, Cr, Fe, Co and Cu, said active material exhibiting topotactic two-phase reactions during charge and discharge.

TECHNICAL FIELD

The present invention relates to a positive electrode-active materialand a non-aqueous electrolyte secondary battery using the same.

BACKGROUND ART

Non-aqueous electrolyte secondary batteries used as power sources formobile communication devices and portable electronic devices in recentyears are characterized by high potential force and high energy density.Examples of positive electrode active materials used for non-aqueouselectrolyte secondary batteries include lithium cobalt oxide (LiCoO₂),lithium nickel oxide (LiNiO₂), manganese spinel (LiMn₂O₄), etc. Theseactive materials have a voltage of not less than 4 V relative to that oflithium. On the other hand, a carbon material is usually used in thenegative electrode, which is combined with the above-mentioned positiveelectrode active material to give a 4V level lithium ion battery.

The need has been increasing for batteries not only with high energydensity, but also with improved high rate characteristics and improvedpulse characteristics. Charging/discharging at a high rate imposes anincreased load on the active material, making it difficult to maintainthe cycle life by conventional techniques.

Some devices require batteries that have such high rate dischargeperformance and yet exhibit a flat battery voltage in thecharge/discharge curves. Batteries with a positive electrode activematerial having a layered structure such as lithium cobalt oxide(LiCoO₂) or lithium nickel oxide (LiNiO₂) usually exhibit relativelyflat S-shaped charge/discharge curves. Accordingly, it is difficult tomaintain a flat charge/discharge voltage during high ratecharging/discharging. Since the positive electrode active materialrepeatedly expands and contracts to a great degree in the layerdirection during charging/discharging, the stress resulting therefromreduces the cycle life particularly at the time of high ratecharging/discharging.

The positive electrode active materials are recognized to haverelatively flat-shaped charge/discharge curves. However, from theviewpoint of determination of the remaining capacity, they areconsidered as not suitable for determining the remaining capacitybecause accurate analysis in a narrow potential range is necessary.Particularly, when lithium is intercalated into the negative electrodeduring charging, the potential of the negative electrode rapidly dropsto about 0.1 V and, after that, the negative electrode absorbs lithiumat a given potential. As for the positive electrode active materials,since LiMn₂O₄ having a spinel structure in particular exhibits flattercharge and discharge curves than lithium cobalt oxide (LiCoO₂) andlithium nickel oxide (LiNiO₂) both having a layered structure, LiMn₂O₄is considered as not suitable for determining the remaining capacity.

In order to determine the remaining capacity of a non-aqueouselectrolyte secondary battery, usually, current and time other thanvoltage are detected and a calculation is then made in an integratedcircuit based on the above information to yield the remaining capacityof a battery, which is typified by, for example, the method of JapaneseLaid-Open Patent Publication No. Hei 11-072544.

In order to monitor the completion of charging, Japanese Laid-OpenPatent Publication No. 2000-348725 proposes to use LiMn₂O₄ as a positiveelectrode active material and Li₄Ti₅O₁₂ and natural graphite as negativeelectrode active materials. This technique enables the monitoring of thecompletion of charging by creating a potential difference in thepotential of the negative electrode. The reference discloses a negativeelectrode comprising Li₄Ti₅O₁₂ whose potential persists at 1.5 V andnatural graphite whose potential persists at 0.1 V.

Now, the following describes a conventionally proposed battery systemcomprising a positive electrode containing a conventional positiveelectrode active material having a spinel structure and a negativeelectrode containing a lithium-containing titanium oxide having a spinelskeleton.

For example, Japanese Laid-Open Patent Publication No. Hei 11-321951proposes a positive electrode active material represented by theformula: Li_((1+x))Mn_((2−x−y))M_(y)O_(z), where 0≦x≦0.2, 0.2≦y≦0.6,3.94≦z≦4.06, and M is nickel or a compound composed of nickel as anessential component and at least one selected from aluminum andtransition elements, and a method for synthesizing the positiveelectrode active material without an impurity of NiO. To be specific, amixture comprising a manganese compound and a metal M compound is bakedat 900 to 1100° C., and the mixture is baked again with a lithiumcompound.

This method, however, involves a reaction between manganese and a metalM, that is, a reaction between solids. Accordingly, it is difficult thatthe above two is incorporated uniformly. In addition, since the bakingis performed at a high temperature of not less than 900° C., reactivitywith lithium is reduced after the baking, making it difficult to obtainthe desired positive electrode active material.

Japanese Laid-Open Patent Publication No. Hei 9-147867 discloses apositive electrode active material comprising an intercalation compoundhaving a spinel crystal structure and being represented by the generalformula: Li_(x+y)M_(z)Mn_(2−y−z)O₄, where M represents a transitionmetal, 0≦x≦1, 0≦y<0.33, and 0<z<1. The disclosed positive electrodeactive material is capable of charging/discharging at a potential of notless than 4.5 V relative to that of Li/Li⁺.

Japanese Laid-Open Patent Publication No. Hei 7-320784 discloses abattery comprising a positive electrode containing Li₂MnO₃ or LiMnO₂ asan active material and a negative electrode containinglithium-intercalated Li_(4/3)Ti_(5/3)O₄ or LiTi₂O₄ as an activematerial. Japanese Laid-Open Patent Publication No. Hei 7-335261discloses a battery comprising a positive electrode containing a lithiumcobalt oxide (LiCoO₂) and a negative electrode containing a lithiumtitanium oxide (Li_(4/3)Ti_(5/3)O₄). Further, Japanese Laid-Open PatentPublication No. Hei 10-27609 discloses a battery comprising: a negativeelectrode containing, as an active material, lithium, a lithium metal,or a lithium-titanium oxide with a spinel-type structure; a positiveelectrode containing, as an active material, a lithium-manganese oxidewith a spinel-type structure (Li_(4/3)Mn_(5/3)O₄); and an electrolytecomprising a solvent mixture of not less than two components such asLiN(CF₃SO₂)₂ and ethylene carbonate.

Japanese Laid-Open Patent Publication No. Hei 10-27626 discloses to usea lithium-containing transition metal oxide (LiA_(x)B_(1−x)O₂) as apositive electrode active material and a lithium-titanium oxide(Li_(4/3)Ti_(5/3)O₄) as a negative electrode active material, and to setthe actual content ratio of the negative electrode active material tothe positive electrode active material to be not greater than 0.5.Japanese Laid-Open Patent Publication No. Hei 10-27627 discloses to usea lithium-manganese oxide (Li_(4/3)Mn_(5/3)O₄) as a positive electrodeactive material, and a lithium-titanium oxide (Li_(4/3)Ti_(5/3)O₄) andlithium in the negative electrode, and to set the molar ratio of thelithium-titanium oxide to the lithium-manganese oxide to be not greaterthan 1.0, and the molar ratio of the lithium to the lithium-titaniumoxide to be not greater than 1.5.

Furthermore, Japanese Laid-Open Patent Publication No. 2001-243952discloses a lithium secondary battery comprising: a positive electrodecontaining a positive electrode active material represented by theformula: Li_(1−x)A_(x)Ni_(1−y)M_(y)O₂, where A is one or more selectedfrom alkali metals except Li and alkali earth metals, M is one or moreselected from Co, Mn, Al, Cr, Fe, V, Ti and Ga, 0≦x≦0.2, and 0.05≦y≦0.5,and comprising secondary particles formed by the aggregation of primaryparticles with a mean particle size of not less than 0.5 μm; and anegative electrode containing, as a negative electrode active material,a lithium-titanium composite oxide represented by the formula:Li_(a)Ti_(b)O₄, where 0.5≦a≦3, and 1≦b≦2.5.

Still furthermore, Japanese Laid-Open Patent Publication No. 2001-210324discloses a battery comprising: a positive electrode containing, as apositive electrode active material, a lithium-manganese composite oxiderepresented by the composition formula: Li_(1+x)M_(y)Mn_(2−x−y)O_(4−z),where M is one or more selected from Ti, V, Cr, Fe, Co Ni, Zn, Cu, W, Mgand Al, 0≦x≦0.2, 0≦y<0.5, and 0≦z<0.2, having a half peak width of the(400) peak of not less than 0.02 θ and not greater than 0.1 θ (θ is anangle of diffraction) obtained from a powdered X-ray diffraction usingCuKα radiation, and whose primary particles are octahedron in shape; anda negative electrode containing, as a negative electrode activematerial, a lithium-titanium composite oxide represented by thecomposition formula: Li_(a)Ti_(b)O₄, where 0.5≦a≦3.1, and 1≦b≦2.5.

Some of the conventional techniques, however, cannot completely solvethe above-mentioned problems such as improving high rate characteristicsand pulse characteristics. For example, charging/discharging at a highrate imposes an increased load on the active material to causestructural damage, thus making it difficult to maintain the cycle life.In addition, since a lithium cobalt oxide and a graphite material, bothhaving a layered structure, repeatedly expand and contract to a greatdegree in the layer direction during charging/discharging, a stress isgiven to the active material and an electrolyte exudes from betweenelectrodes, thus reducing the cycle life particularly at the time ofhigh rate charging/discharging. Accordingly, in order to extend thecycle life of such batteries, it is important to prevent the expansionand contraction of the active material.

Batteries used as power sources for electronic devices preferablyexhibit a flat-shaped discharge curve, and are required to exhibit aflat voltage even during such high rate discharging. However, batteriescurrently in practical use exhibit either an S-shaped discharge curve inwhich the voltage gradually decreases, or a flat discharge curve inwhich the battery voltage suddenly decreases at the end of charging. Theformer has the problem that it should have a flatter voltage although itis not difficult to monitor the remaining capacity thereof. In the caseof the latter, on the other hand, the voltage difference is extremelysmall until the end of discharging, so that it is very difficult tomonitor the remaining capacity of the battery. Accordingly, obtaining abattery whose remaining capacity can be moderately monitored remains oneof the problems.

In view of the above, an object of the present invention is to solvethese problems. To be specific, an object of the present invention is toprovide a non-aqueous electrolyte secondary battery with improved ratecharacteristics, improved cycle life, improved safety and improvedstorage life designed by optimizing the composition and crystalstructure of a positive electrode active material, a method forsynthesizing the above, the selection of battery systems, anelectrolyte, current collector materials for positive and negativeelectrodes, a separator, the content ratio between positive and negativeelectrode active materials, and the like. The present invention furtherprovides a non-aqueous electrolyte secondary battery containing apositive electrode active material having flat charge/discharge curvesand whose remaining capacity can be easily monitored by deliberatelycreating a voltage difference at the end of discharging.

DISCLOSURE OF INVENTION

1. Positive Electrode Active Material

The present invention relates to a positive electrode active materialrepresented by the composition formula: Li_(2±α)[Me]₄O_(8−x), where0≦α<0.4, 0≦x<2, and Me is a transition metal containing Mn and at leastone selected from the group consisting of Ni, Cr, Fe, Co and Cu, thematerial exhibiting topotatic two-phase reactions during charge anddischarge.

The phase of the transition metal preferably has a 2×2 superlattice inthe positive electrode active material.

It is preferred that the ratio between Mn and other transition metal issubstantially 3:1 in the positive electrode active material.

It is preferred that the positive electrode active material has aspinel-framework-structure with space group symmetry of Fd3m in which Liand/or Me locate at the 16(c) sites.

The difference between the charge and discharge potentials of thepositive electrode active material is preferably 0.2 to 0.8 V.

The positive electrode active material preferably has a lattice constantattributed to a cubic crystal of not greater than 8.3 Å.

Preferably, the positive electrode active material is not only in theform of an octahedral shape. In other words, the positive electrodeactive material particles are preferably in the form oficositetrahedron, rhombic dodecahedron, or tetradecahedron with 8hexagons and 6 quadrangles.

The positive electrode active material preferably comprises a mixture ofcrystal particles with a particle size of 0.1 to 8 μm and secondaryparticles of the crystal particles with a particle size of 2 to 30 μm.

2. Method for Producing Positive Electrode Active Material

The present invention relates to a method for producing a positiveelectrode active material comprising: (1) a step of mixing Mn and acompound containing at least one selected from the group consisting ofNi, Cr, Fe, Co and Cu to give a raw material mixture; or a step ofsynthesizing a eutectic compound containing a Mn compound and at leastone selected from the group consisting of Ni, Cr, Fe, Co and Cu; (2) astep of mixing the raw material mixture or eutectic compound with alithium compound; and (3) a step of subjecting the compound obtained bythe step (2) to a first baking at a temperature of not less than 600°C., whereby a positive electrode active material represented by theformula: Li_(2±α)[Me]₄O_(8−x), where 0≦α<0.4, 0≦x<2, and Me is atransition metal containing Mn and at least one selected from the groupconsisting of Ni, Cr, Fe, Co and Cu, said material exhibiting topotatictwo-phase reactions during charge and discharge is obtained.

The first baking is preferably performed at a temperature of not lessthan 900° C.

The production method preferably comprises a step of performing a secondbaking at a temperature lower than that of said first baking after saidfirst baking.

In this case, the second baking is preferably performed at a temperatureof 350 to 950° C.

More preferably, the second baking is performed at a temperature of 650to 850° C.

Still more preferably, the production method further comprises a step ofrapidly cooling the positive electrode active material after the firstbaking and/or the second baking.

The rapid cooling is preferably performed at a temperature decrease rateof not less than 4.5° C./min, more preferably at a temperature decreaserate of not less than 10° C./min.

The rapid cooling is preferably performed until the temperature reachesroom temperature.

3. Non-Aqueous Electrolyte Secondary Battery

The present invention further relates to a non-aqueous electrolytesecondary battery comprising a positive electrode containing theabove-described positive electrode active material, a negative electrodecontaining a titanium oxide, a non-aqueous electrolyte and a separator,characterized in that the battery has a practical charging/dischargingregion of 2.5 to 3.5 V and a practical average voltage of 3V level.

The titanium oxide preferably has a spinel structure.

The titanium oxide is preferably Li₄Ti O₁₂.

The non-aqueous electrolyte secondary battery has a potential differenceof 0.2 to 0.8 V in an operating discharge voltage.

The positive and negative electrodes preferably have a current collectormade of aluminum or an aluminum alloy.

The non-aqueous electrolyte preferably comprises at least one selectedfrom the group consisting of propylene carbonate, γ-butyrolactone,γ-valerolactone, methyl diglyme, sulfolane, trimethyl phosphate triethylphosphate and methoxymethylethyl carbonate.

The separator is preferably made of non-woven fabric.

The non-woven fabric preferably comprises at least one selected from thegroup consisting of polyethylene, polypropylene and polybutyleneterephthalate.

The weight ratio of the negative electrode active material to thepositive electrode active material is preferably not less than 0.5 andnot greater than 1.2.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows graphs illustrating the electrochemical characteristics ofpositive electrode active materials obtained by baking the mixture of aeutectic compound and a lithium compound in air at temperatures of 1000°C. (a), 900° C. (b), 800° C. (c), 700° C. (d) and 600° C. (e) for 12hours (first baking).

FIG. 2 shows the TG curve (thermogravimetric analysis) of a positiveelectrode active material after the first baking.

FIG. 3 shows the charge/discharge curves of a positive electrode activematerial in accordance with the present invention obtained by the firstbaking at 1000° C. for 12 hours and then the second baking at 700° C.for 48 hours.

FIG. 4(a) shows the SEM image of a positive electrode active material inaccordance with the present invention in cross section and FIG. 4(b)shows the SEM image of a conventional positive electrode active materialin cross section.

FIG. 5 shows the SEM image of positive electrode active materialparticles produced under the conditions of Case 1.

FIG. 6 shows the SEM image of positive electrode active materialparticles produced under the conditions of Case 2.

FIG. 7 shows the SEM images of positive electrode active materialparticles produced under the conditions of Case 3.

FIG. 8 shows the SEM images of positive electrode active materialparticles produced under the conditions of Case 4.

FIG. 9 shows the X-ray diffraction patterns of the positive electrodeactive materials in accordance with the present invention produced atdifferent temperatures in the first baking.

FIG. 10 shows the FT-IR analysis results of the positive electrodeactive materials in accordance with the present invention produced atdifferent temperatures in the first baking.

FIG. 11 shows the X-ray diffraction patterns of the positive electrodeactive materials in accordance with the present invention produced undervarious different conditions.

FIG. 12 shows the FT-IR analysis results of the positive electrodeactive materials in accordance with the present invention synthesizedunder various different conditions.

FIG. 13 shows a graph illustrating the occupancy of elements in each ofthe atomic sites of the crystal structure of a positive electrode activematerial in accordance with the present invention.

FIG. 14 shows the lattice constant change of a positive electrode activematerial produced through a rapid cooling.

FIG. 15 shows the X-ray diffraction patterns of the positive electrodeactive material in accordance with the present invention duringcharging/discharging.

FIG. 16 shows the lattice constant change of a positive electrode activematerial during charging/discharging.

FIG. 17 shows the charge/discharge behavior of a battery system inaccordance with the present invention.

FIG. 18 shows the cycle life of a battery system in accordance with thepresent invention until 200 cycles.

FIG. 19 shows the rate capability of a battery system with load inaccordance with the present invention.

FIG. 20 shows the high rate discharge characteristics (without adifference) of a battery system in accordance with the presentinvention.

FIG. 21 shows the expansion and contraction during charging/dischargingmeasured by a dilatometer.

FIG. 22 shows the electrochemical behaviors of the positive electrodeactive materials in accordance with the present invention producedthrough a rapid cooling.

FIG. 23 shows the discharge behavior of a battery system in accordancewith the present invention.

FIG. 24 shows the high rate characteristics of a battery system inaccordance with the present invention.

FIG. 25 shows the pulse discharge characteristics of a battery system inaccordance with the present invention.

FIG. 26 shows a front view, in vertical cross section, of a cylindricalbattery produced in examples of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention can provide a non-aqueous electrolyte secondarybattery with a flat charge/discharge voltage, excellent high ratecharacteristics and excellent cycle life by optimizing design matterssuch as a new composition of a positive electrode active material, a newmethod for synthesizing a positive electrode active material, materialsfor battery elements other than a positive electrode active material,and the content ratio between positive and negative electrode activematerials.

If an appropriate battery system is designed using a positive electrodeactive material of the present invention, a potential difference can befreely created at around the end of discharging. Thereby, it is possibleto accurately determine the remaining capacity of a non-aqueouselectrolyte secondary battery in accordance with the present invention,and to add an alarm function that accurately informs a loss of powercapacity.

Since a positive electrode active material in accordance with thepresent invention exhibits a flat-shaped discharge curve, by using, forexample, Li₄Ti₅O₁₂ which exhibits a flat-shaped discharge curve in thenegative electrode, it is possible to obtain a battery that exhibits aflat-shaped discharge curve desirable for electronic devices.

In addition, since such battery in accordance with the present inventionprovides a voltage of 3V level, it can be utilized in devices such ascameras, digital cameras, game machines, portable MD players and headsetstereos instead of a conventional lithium primary battery or aconventional combination of two dry batteries, whereby remarkable effectis obtained.

1. Synthesis of Positive Electrode Active Material of the PresentInvention

The present invention relates to a positive electrode active materialrepresented by the composition formula: Li_(2±α)[Me]₄O_(8−x), were0≦α<0.4, 0≦x<2, and Me is a transition metal containing Mn and at leastone selected from the group consisting of Ni, Cr, Fe, Co and Cu, thematerial exhibiting topotatic two-phase reactions during charge anddischarge. The composition formula preferably satisfies 0≦x<1.3.

The following explains a positive electrode active material inaccordance with the present invention using Li[Ni_(1/2)Mn_(3/2)]O₄ as arepresentative example. It is to be understood that the explanation canalso be applied to a positive electrode active material with a differentcomposition within the scope of the aforesaid formula.

Li[Ni_(1/2)Mn_(3/2)]O₄ can be synthesized by mixing raw materials suchas an oxide containing constituent elements, a hydroxide and/or acarbonate at a desired composition to give a mixture, which is thenbaked (first baking). In this case, however, it is necessary to make theparticles of the materials all the same size and to thoroughly mix themin order to achieve a uniform reaction. Besides, the synthesis requiresan advanced powder technology.

At the same time, Li[Ni_(1/2)Mn_(3/2)]O₄ can also be synthesized bycoprecipitating nickel and manganese in an aqueous solution in the formof hydroxide or carbonate. In this case, the synthesis can be relativelyeasily performed because it is possible to uniformly disperse nickel andmanganese, both of which are unlikely to be dispersed, in the particlebeforehand.

Accordingly, in the examples of the synthesis thereof described below,an eutectic compound obtained as a hydroxide is used and lithiumhydroxide is used as a lithium compound. After they are thoroughlymixed, the obtained mixture is baked (first baking). It is also possibleto ensure the reaction by forming the mixture of hydroxide obtainedthrough a eutectic reaction and lithium hydroxide into pellets.

FIG. 1 shows graphs illustrating the electrochemical characteristics ofthe positive electrode active materials obtained by baking the mixtureof the eutectic compound and the lithium compound in air at temperaturesof 1000° C. (a), 900° C. (b), 800° C. (c), 700° C. (d) and 600° C. (e)for 12 hours (first baking). Specifically, [Ni_(1/4)Mn_(3/4)](OH)₂obtained through a eutectic reaction and LiOH.H₂O were thoroughly mixedto give a mixture, which was then formed into pellets and the resultingformed product was baked to give a Li[Ni_(1/2)Mn_(3/2)]O₄.

A test cell was produced as follows and the electrochemicalcharacteristics thereof were measured.

First, 80 parts by weight of Li[Ni_(1/2)Mn_(3/2)]O₄, 10 parts by weightof acetylene black as a conductive agent and 10 parts by weight ofpolyvinylidene fluoride (PVDF) as a binder were mixed to give a mixture,which was then diluted with N-methyl-2-pyrrolidone (NMP) to give apaste. The paste was applied onto a current collector made of aluminumfoil. The current collector with the paste applied was dried in a vacuumat 60 for 30 minutes, which was then cut into 15 mm×20 mm pieces.Subsequently, the cut piece of current collector was further dried in avacuum at 150° C. for 14 hours to give a test electrode.

A counter electrode was produced by attaching a lithium metal sheet on astainless steel. As a separator, a porous film made of polyethylene wasused. An electrolyte solution was obtained by mixing ethylene carbonate(EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7, and 1.0 M ofLiPF₆ was then dissolved in the obtained solvent mixture.

The test electrode, the separator and the metal lithium were stacked inthis order. After the electrolyte solution was fed thereinto, the stackwas clamped between holders made of stainless steel by applying anappropriate pressure from outside to give a test cell. The obtained testcell was repeatedly charged and discharged between 3.0 to 5.0 V at acurrent density of 0.17 mA/cm².

As is apparent from FIG. 1, the positive electrode active materialsobtained by baking at any of the temperatures had a high dischargevoltage of 4.6 to 4.8 V relative to that of lithium metal and adischarge capacity of about 125 mAh/g. It also shows that the higher thebaking temperature was, the more excellent the polarizationcharacteristics were.

It is also clear that the voltage difference at around 4 V regularlyincreased as the baking temperature was increased. The present inventionexploits such phenomenon to provide a battery suitable for electronicdevices and whose remaining capacity can be detected. In other words, itis possible to control the desired timing for detecting the remainingcapacity by changing the baking temperature. The difference appeared ataround 4 V, and the range was 0.2 to 0.8 V, which was not as remarkablea change as several V. Accordingly, if a battery with such a differenceis used in an electronic device, such trouble as the shutdown of thepower source of the electronic device will not occur.

As described above, a positive electrode active material in accordancewith the present invention can be produced by mixing a raw materialmixture or a eutectic compound with a lithium compound, which is thensubjected to a first baking and subsequently the ambient temperature isgradually decreased (gradual cooling). The conditions for the firstbaking and the gradual cooling are as follows.

First baking

-   -   Lower limit: 600° C., preferably 900° C.    -   Upper limit: 1000° C.    -   Time: 2 to 72 hours

Cooling rate

-   -   Lower limit: 4.5° C./min.    -   Upper limit: 10° C./min.        2. Control of Voltage Difference at the End of Discharge and        Improvement of Polarization Characteristics in Positive        Electrode Active Material

As the baking temperature is higher, the degree of the polarization getssmaller as previously stated. In this case, however, the range of the 4Vregion becomes greater. Certainly, it is preferred to freely control therange of the 4V region while the polarization is controlled to be small.In order to achieve this object, the present inventors extensivelystudied the synthesis method.

FIG. 2 shows the TG curve (thermogravimetric analysis) of a positiveelectrode active material after the first baking. The positive electrodeactive material used here was obtained by baking Li[Ni_(1/2)Mn_(3/2)]O₄at a low temperature of 500° C. This positive electrode active materialwas heated with the temperature increased by 50° C. from 700 to 850° C..The positive electrode active material was held at each temperature andthe temperature was increased stepwise. When the temperature wasdecreased, the temperature was controlled in the same manner. Thetemperature increase rate was 10° C./min. and the ambient atmosphere wasair.

In FIG. 2, “a” represents temperature, “b” represents weight change whenthe temperature was increased, and “c” represents weight change when thetemperature was decreased. In FIG. 2, there is observed a random weightreduction, which is considered to be due to moisture. In the temperatureincrease from 400 to 1000° C., the weight monotonously decreased in therange of 700 to 1000° C.. On the other hand, when the weight change whenthe temperature was decreased is observed, the weight was increased(recovered) in an amount equal to the amount of weight decreased,following the rate of this experiment. It is apparent that the weightwas almost completely recovered although the rate was slower until 700°C. This weight increase is presumed to be because oxygen once releasedat a high temperature returned to the positive electrode active materialby re-baking (second baking), in other words, by reoxidation of thepositive electrode active material. Accordingly, it suggests that thetemperature of the positive electrode active material obtained after thefirst baking is preferably decreased at a rate of not greater than 10°C./min.

Then, the charge/discharge curves of the positive electrode activematerial obtained by the first baking at 1000° C. for 12 hours and thenthe second baking at 700° C. for 48 hours are shown in FIG. 3. Theresults show that the positive electrode active material has acharge/discharge capacity of about 135 mAh/g, a voltage difference ofabout 15 mAh/g at around 4 V, and excellent polarizationcharacteristics.

As described above, it is even possible to control the voltagedifference at around 4 V in the positive electrode active material,which is obtained by baking at a high temperature of 1000° C. (firstbaking), by re-baking (second baking) the positive electrode activematerial, for example, at a lower temperature of 700° C. like thepositive electrode active material baked at 700° C. as shown in FIG. 1.

Since the positive electrode active material subjected to the first andsecond bakings has been baked at 1000° C. once, it possesses growncrystalline particles having no micropores and therefore has a highpacking density. In addition, this positive electrode active material issuperior in polarization characteristics.

As described above, from the viewpoint of voltage difference andpolarization characteristics, the positive electrode active material inaccordance with the present invention is preferably produced by mixing araw material mixture or a eutectic compound with a lithium compound,which is then subjected to the first baking and then the second baking.The conditions for the first baking and the second baking are asfollows.

First baking

-   -   Lower limit: 600° C., preferably 900° C.    -   Upper limit: 1000° C.    -   Time: 2 to 72 hours

Second baking

-   -   Lower limit: 350° C., preferably 650° C.    -   Upper limit: 950° C., preferably 850° C.    -   Time: 2 to 72 hours

From the results and the electrochemical characteristics evaluationshown in FIG. 2, it is clear that it is preferred that the first bakingis performed at 600 to 1000° C. or more, preferably 900 to 1000° C., thetemperature is rapidly cooled to 350 to 950° C., and then the secondbaking is performed at 350 to 950° C., preferably 650 to 850° C.

It is possible to improve the polarization characteristics of thepositive electrode active material to be obtained and, at the same time,to appropriately control the difference that appears at around 4 V inthe charge/discharge curves. In the above experiment, the temperatureincrease rate during baking was 7.5° C./min. and the temperaturedecrease rate was 4.5° C./min.

(3) Control of Particle Morphology of Active Material

When the positive electrode active material is applied to a battery, theparticle morphology of the positive electrode active material is animportant factor, and it is no exaggeration to state that the control ofthe particle morphology thereof influences the improvement of lithiumion batteries currently available in capacity and performance.

In view of this, the present inventors extensively studied the preferredparticle morphology of the positive electrode active material inaccordance with the present invention and the control of the particlemorphology thereof. As previously stated, in the method for producingthe positive electrode active material in accordance with the presentinvention, it is preferred to perform the first baking at a hightemperature (not less than 900° C.) and then the second baking intendedfor reoxidation.

Accordingly, a cross-section image of a particle of the positiveelectrode active material in accordance with the present inventionobtained by the first baking at 1000° C. for 12 hours and the secondbaking at 700° C. for 48 hours was taken by SEM. The obtained SEM imageis shown in FIG. 4(a) (magnification: 300000 times). FIG. 4(b) shows anSEM image of a positive electrode active material obtained in the samemanner as the positive electrode active material of FIG. 4(a) wasobtained, except that the second baking was not performed.

From FIG. 4, it is clear that the crystalline particle of the positiveelectrode active materials is well grown because they were once baked at1000° C. It is also clear that the particle has no micropores insidethereof, and therefore is a particle with a high packing densityalthough it is a primary particle with a size of 2 to 3 μm.

Further, the particle morphology of the positive electrode activematerial (the external shape in particular) significantly influences acoating density and a packing density when an electrode plate isproduced using the positive electrode active material. JapaneseLaid-Open Patent Publication No. 2001-210324 provides a proposalregarding particle morphology. Specifically, it teaches that the shapeof primary particle should be an octahedron. The positive electrodeactive material in accordance with the present invention preferably hasa shape totally different from an octahedral shape, which will beexplained below, and therefore is obviously different from the aboveprior art in this regard.

First, the method for controlling the particle morphology is explainedusing the case of producing Li[Ni_(1/2)Mn_(3/2)]O₄ as a representativeexample. It is to be noted that positive electrode active materialshaving other compositions within the scope of the present invention alsoexhibited a similar tendency.

(i) Case 1 (FIG. 5)

The first baking was performed by increasing the temperature from roomtemperature to 1000° C. for about 3 hours and holding the temperature at1000° C. for 12 hours. After the first baking, the temperature wasdecreased from 1000° C. to room temperature for 2 hours (cooling rate of8° C. /min.).

(ii) Case 2 (FIG. 6)

The first baking was performed by increasing the temperature from roomtemperature to 1000° C. for about 3 hours and holding the temperature at1000° C. for 12 hours. The second baking was performed by decreasing thetemperature from 1000 to 700° C. for 30 minutes and holding thetemperature at 700° C. for 48 hours.

After the second baking, the temperature was decreased from 700° C. toroom temperature for 1.5 hours (cooling rate of 7.5° C./min.).

(iii) Case 3 (FIG. 7)

The first baking was performed by increasing the temperature from roomtemperature to 1000° C. for about 3 hours and holding the temperature at1000° C. for 12 hours. After the first baking, the temperature wasrapidly cooled from 1000° C. to room temperature.

The second baking was performed by increasing the temperature to 700° C.for about 1 hour and holding the temperature at 700° C. for 48 hours.

After the second baking, the temperature was decreased from 7000° C. toroom temperature for 1.5 hours.

(iv) Case 4 (FIG. 8)

The first baking was performed by increasing the temperature from roomtemperature to 1000° C. for about 3 hours and holding the temperature at1000° C. for 12 hours. After the first baking, the temperature wasrapidly cooled from 1000° C. to room temperature.

Roughly classified, Cases 3 and 4 included a rapid cooling step whileCases 2 and 3 included a reoxidation (second baking) step at 700° C.

FIGS. 5 to 8 show the SEM images of the particulate positive electrodeactive materials produced under the conditions of Cases 1 to 4,respectively. As is obvious from these SEM images, the particulatepositive electrode active materials in accordance with the presentinvention are not in the form of an octahedron. Although it is difficultto identify such shape, it can be said that the positive electrodeactive materials are in the form of an icositetrahedron or rhombicdodecahedron. More specifically, it can be said that the positiveelectrode active materials are in the form of a tetradecahedron with 8hexagons and 6 quadrangles. In FIGS. 7 and 8, the magnification of (a)was 1000 times and that of (b) was 30000 times.

It is clear that the rapid cooling step significantly influences thecontrol of the particle morphology. The particle boundary of thepositive electrode active materials obtained form Cases 1 and 2 is sharpwhereas that of the positive electrode active materials obtained fromCases 3 and 4 is curved. This indicates that the boundary became curvedby performing the rapid cooling step.

When the positive electrode active materials obtained from Cases 1 to 4were applied onto electrode plates for batteries, the use of thepositive electrode active material with curved boundary enabled highdensity filling because the fluidity of powder or applied paste wasimproved. As described above, unlike the particles in the form of anoctahedron proposed by the prior art, the positive electrode activematerial in accordance with the present invention is in the form oficositetrahedron or rhombic dodecahedron, more specifically, in the formof a tetradecahedron with 8 hexagons and 6 quadrangles. Presumably, thisparticle morphology contributes to achieve improved batterycharacteristics. The positive electrode active material in accordancewith the present invention preferably comprises a mixture of crystalparticles with a particle size of about 0.1 to 8 μm and secondaryparticles of the crystal particles with a particle size of 2 to 30 μm.

As described above, from the viewpoint of controlling the particlemorphology, the positive electrode active material in accordance withthe present invention is preferably produced by mixing a raw materialmixture or a eutectic compound with a lithium compound, which is thensubjected to a first baking and a rapid cooling. Additionally, a secondbaking may be performed after the rapid cooling. The conditions for thefirst baking, the rapid cooling and the second baking are as follows.

First baking

-   -   Lower limit: 600° C., preferably 900° C.    -   Upper limit: 1000° C.    -   Time: 2 to 72 hours

Cooling rate

-   -   10° C./min. or more,    -   preferably 20° C./min. or more,    -   most preferably 50° C./min. or more

Second baking

-   -   Lower limit: 350° C., preferably 650° C.    -   Upper limit: 950° C., preferably 850° C.    -   Time: 2 to 72 hours        (4) Crystal Structure, X-Ray Diffraction Pattern and FT-IR        Signal of Positive Electrode Active Material

In terms of crystal structure, the positive electrode active material inaccordance with the present invention has a spinel-framework-structure.FIG. 9 shows the X-ray diffraction patterns of the positive electrodeactive materials in accordance with the present invention produced atdifferent temperatures in the first baking. In FIG. 9, (a) to (e) showthe X-ray diffraction patterns of the positive electrode activematerials produced by the first baking at 600° C., 700° C., 800° C.,900° C. and 1000° C., respectively. The composition of the positiveelectrode active material was Li [Ni_(1/2)Mn_(3/2)]O₄.

When Miller indices were assigned as cubic crystal in the obtained X-raydiffraction patterns, all peaks were able to be assigned as shown inFIG. 9. From FIG. 9, it is found that the peaks in the case of the firstbaking at a high temperature are sharper, exhibiting the improvement incrystallinity.

Then, the FT-IR analysis results of the positive electrode activematerial shown by (a) to (e) in FIG. 9 are shown by (a) to (e) in FIG.10. Eight sharp peaks are observed in the case (b) of the positiveelectrode active material obtained at 700° C., and it is obvious thatthe peaks become broad in both cases of over 700° C. and less than 700°C.. This indicates that the fist baking at 700° C. is preferred in termsof crystal arrangement.

Regardless of performing the rapid cooling or not, almost similar X-raydiffraction patterns can be obtained by the second baking. In order tostudy the crystal structure in more detail, the X-ray diffractionpatterns of the positive electrode active materials obtained from Cases3 and 4 described previously were compared.

FIG. 11 shows the X-ray diffraction patterns of the positive electrodeactive material (a) obtained from the above Case 3 and the positiveelectrode active material (b) obtained from the above Case 4, and FIG.12 shows their FT-IR analysis results. The difference between them iswhether they were subjected to the reoxidation process or not. In viewof this, the structural analysis of the X-ray diffraction pattern of therapidly cooled sample was performed. As a result, the following hasbecome clear and this is thought to be the cause of the effect of thepresent invention.

It can be said that the positive electrode active material in accordancewith the present invention has the formula: Li_(2±α)[Me]₄O_(8−x), where0≦α<0.4, 0≦x<2, preferably 0≦x<1.3, and Me is a transition metalcontaining Mn and at least one selected from the group consisting of Ni,Cr, Fe, Co and Cu. In the following, an explanation is given using aspecific example in which a is set to 0 (α=0) in order to make it easierto understand.

In the atomic arrangement of the spinel structure belonging to the spacegroup symmetry of Fd3m of LiMn₂O₄, lithium element occupies the 8asites, a transition metal element Me (Mn) occupies the 16d sites andoxygen occupies the 32e sites. However, the 16c sites are usuallyvacant. The positive electrode active material in accordance with thepresent invention is characterized by arranging an element in the 16csites.

In other words, in the positive electrode active material of the presentinvention, the control of the voltage difference in the above dischargecurve is achieved by controlling the amount of an element to be presentin the 16c sites.

When the X-ray diffraction pattern of the sample of the positiveelectrode active material produced by the first baking and the rapidcooling (without the second baking, in other words, without reoxidation)is analyzed, it was found that the X-ray diffraction pattern could bewell fitted by assuming that Me was present in the 8a sites at about1/5, in the 16c sites at 2/5 and in the 16d sites at 7/4. From this, itis presumed that the oxygen in the spinel structure leaves as thetemperature is increased to 1000° C., thereby the transition metal isreduced and considerable amounts of lithium element and transition metalelement respectively move to the 8a sites and 16c sites. Because of thisphenomenon, a rock-salt-type-structure is formed in a part of the spinelstructure of the positive electrode active material in accordance withthe present invention.

Since the above sample obtained by the first baking and the rapidcooling was not subjected to the reoxidation process for reinjection ofoxygen, it can be represented by Li_(1.2)Me_(2.4)O₄, judging from theresult of the above-mentioned TG curve. Me contains Ni and Mn in a ratioof 1:3.

Further, the X-ray diffraction patterns shown in FIG. 11 indicate thatthe rock-salt-type-structures reversibly return to the spinel structuresby the reoxidation (second baking) in the positive electrode activematerial (a) which was obtained by the first baking, the rapid coolingand the reoxidation (second baking) at 700° C., and the positiveelectrode active material (c) which was obtained by the first baking(1000° C.), and subsequently the reoxidation (second baking) at a lowertemperature of 700° C.. Such flexible crystal structures of the positiveelectrode active material contribute to the stability of the crystalstructure in the case where a stress is given to the positive electrodeactive material due to high rate charge/discharge cycles; as a result,presumably a long life can be achieved.

Further, in the FT-IR analysis results shown in FIG. 12, eight peaks areclearly observed in the case of the positive electrode active materials(a) and (c) obtained through the reoxidation (second baking) process.

Contrary to the above, the X-ray diffraction patterns and FT-IR analysisresults of the positive electrode active materials obtained simply bythe first baking, which were reoxidized when the temperature wasdecreased are respectively shown by (b) and (d) in FIGS. 11 and 12. Fromthe X-ray diffraction patterns of these positive electrode activematerials, it seems that these positive electrode active materials alsohave a similar spinel-framework-structure to the positive electrodeactive material obtained by the reoxidation (second baking). However,the FT-IR analysis results are distinctly different because eight peakscannot be observed clearly. Also, such eight peaks cannot theoreticallypredicted from the local symmetry of the Fd3m of the spinel structure.Accordingly, it is possible to identify the positive electrode activematerial in accordance with the present invention by the FT-IR analysisresults. This method is effective in identifying the positive electrodeactive material whose charge/discharge curves substantially have novoltage difference.

Now, an explanation is given on a and x values in the compositionformula: Li_(2±α)[Me]₄O_(8−x), where 0≦α<0.4, 0≦x<2, preferably 0≦x<1.3,and Me is a transition metal containing Mn and at least one selectedfrom the group consisting of Ni, Cr, Fe, Co and Cu.

α value is an element to be changed to control the particle growth. Ifthe a value is less than 2 in the stoichiometric composition, it ispossible to control the particle growth during the synthesis, and thesurface area is likely to be increased. Conversely, if the a value isgreater than that, it is possible to facilitate the particle growth.Accordingly, in the case of designing the particles according to thecharacteristics required for a battery, the particle growth can becontrolled by changing the composition ratio of lithium. The range ofthe a value is substantially about ±0.4. If the range (range ofvariation) exceeds this value, the inherent function of the positiveelectrode active material could be harmed.

On the other hand, as previously stated, since the positive electrodeactive material obtained by the first baking at 1000° C. and the rapidcooling is represented by Li_(1.2)Me_(2.4)O₄, the x value can becalculated to be 1.33. The x can be considered to be 2 because theamount of oxygen returns to the stoichiometric composition byreoxidation (second baking). However, the upper limit of the x ispractically 1.3. In view of these facts, especially the fact that oxygenreturns by the reoxidation, the present inventors set the x range to be0≦x<1.3.

Now, the occupancy of each of the atomic sites in the crystal structureof the positive electrode active material in accordance with the presentinvention is shown in FIG. 13. FIG. 13 is a graph schematically showingthat the x value versus the occupancy of the elements in each of thesites. As shown in FIG. 13, it is possible to freely control the voltagedifference that appears in the discharge curve by introducing each ofthe elements into each site to effectively use originally vacant sites.

In view of this and the analysis results of XAFS and the like together,it is considered that the voltage difference in the 4V region isattributed to the electrochemical reaction: Mn³⁺→Mn⁴⁺ and the voltagedifference in the 5V (4.7 V) region is attributed to the electrochemicalreaction: Ni²⁺→Ni⁴⁺. It was found that the voltage difference of theabove-mentioned two positive electrode active material samples obtainedthrough the rapid cooling can also be freely controlled by performingthe reoxidation process a few times after the rapid cooling.

When identifying the positive electrode active material in accordancewith the present invention by the X-ray diffraction patterns or the unitlattice, the following points are noted from the above explanation. Inorder to obtain a positive electrode active material with little voltagedifference (i.e. practically almost unobservable voltage difference), itis preferable to consider the following points.

FIG. 14 shows the lattice constant change of the positive electrodeactive material, which is produced through the rapid cooling. From thisfigure, it is concluded that the preferred lattice constant is notgreater than 8.33 Å, more preferably not greater than 8.25 Å, mostpreferably not greater than 8.2 Å.

From the capacity and the shape of the discharge curves, it was foundthat the most preferred ratio between Mn and the other transition metalelement was substantially 3:1. Although the specific cause thereof isnot known, it is presumed that, when the ratio is 3:1, the transitionmetal phases in the framework of the spinel structure can form asuperlattice of [2×2] and this effect has some influence thereon. Fromthe electron beam diffraction analysis, superlattice spots are observedin this direction, so that the formation of the superlattice of [2×2]can be confirmed.

Although Japanese Laid-Open Patent Publication No. Hei 9-147867 providesa description on high voltage positive electrode active materials, itonly discloses the composition and simple structure thereof and there isno disclosure on the preferred production method and temperature range.To be specific, it only discloses that raw materials are simply mixedand then baked, and broad baking temperatures. The positive electrodeactive material in accordance with the present invention, on the otherhand, has an effect superior to the positive electrode active materialobtained based on such prior art and therefore is a novel material. Tofreely control the particle morphology by the conditions in theproduction method as the present invention suggests is not disclosedeven in Japanese Laid-Open Patent Publication No. Hei 9-147867.

Looking at the crystal structure in particular, Japanese Laid-OpenPatent Publication No. Hei 9-147867 states that Mn in LiMn₂O₄ having anideal spinel structure is substituted with a transition metal or Li.This description is focused only on 16d sites and the specificationclearly states, in the body thereof, that the invention differssignificantly from LiNiVO₄ and the like. This means, in other words,that Japanese Laid-Open Patent Publication No. Hei 9-147867 describesthat atoms are not present in the 8a sites and originally vacant 16dsites.

Contrary to the above, in the present invention, these sites areutilized to form a rock-salt-type-structure in a part of the positiveelectrode active material by appropriately controlling the conditionsfor the production method, and the structure is intentionally controlledby reoxidation (second baking). In short, a rock-salt-type-structure anda spinel-framework-structure are allowed to exist in the same crystal,and the ratio thereof is freely controlled. Additionally, in the case ofthe positive electrode active material with almost onlyspinel-framework-structures and whose discharge curve does notsubstantially have a voltage difference, the signal for identificationis whether eight peaks are clearly observed in the FT-IR analysis ornot.

(5) Topotactic Two-Phase Reaction

Batteries that exhibit a flat discharge curve are more advantageous fordevices to be used. Generally, when the charge/discharge reaction of thepositive electrode active material occurs in one phase, the dischargecurve has the shape of S according to Nernst's equation. Althoughtopotactic two-phase reactions proceed partially in a layered structurematerial such as lithium cobalt oxide or lithium nickel oxide, one-phasereactions proceed most of the time. Accordingly, a layered structurematerial inherently exhibits an S-shaped discharge curve. For thisreason, a significant voltage drop occurs with polarization particularlyat the end of high rate discharging, making it difficult to obtain aflat discharge curve.

When the charging/discharge of the positive electrode active materialproceeds as two-phase reaction, the discharge curve is inherently flat.Accordingly, a positive electrode active material having topotactictwo-phases throughout the charging/discharging reaction is preferred.FIG. 15 shows the X-ray diffraction patterns of the positive electrodeactive material in accordance with the present invention duringcharging/discharging. In FIG. 15, (a) to (m) represent the case of 15mAh/g, 30 mAh/g, 50 mAh/g, 60 mAh/g, 70 mAh/g, 75 mAh/g, 80 mAh/g, 90mAh/g, 100 mAh/g, 105 mAh/g, 110 mAh/g, 120 mAh/g and 136.3 mAh/g. InFIG. 15, splits are observed in the peak changes at (111), (311) and(400), which indicates the fact that topotactic two-phase reactionsproceed in the positive electrode active material.

In order to facilitate the understanding, the change in lattice constantobtained from FIG. 15 when assigned as cubic crystal is shown in FIG.16. The lattice constant in the portion where two lattice constantsexist is calculated assuming that the positive electrode active materialhas two phases.

FIG. 16 illustrates that the discharge of the positive electrode activematerial in accordance with the present invention can be divided intothe first half and the latter half, and topotactic two-phase reactionsproceed in either case. In conventional LiMn₂O₄ with a spinel structure,topotactic two-phase reactions proceed in the first half of discharging,but one-phase reactions proceed in the latter half of discharging.Accordingly, topotactic two-phase reactions do not proceed throughoutdischarging. Unlike the conventional material, in the positive electrodeactive material in accordance with the present invention, topotactictwo-phase reactions proceed throughout discharging to exhibit a flat andextremely good discharge curve.

(6) 3V Level Non-Aqueous Electrolyte Secondary Battery with OxideNegative Electrode and Detection of Remaining Capacity

A description is given on the advantage of a non-aqueous electrolytesecondary battery in which the positive electrode active material inaccordance with the present invention is used in the positive electrodeand a titanium oxide having a spinel structure is used in the negativeelectrode. The positive electrode active material in accordance with thepresent invention has a greater reversible capacity and betterpolarization characteristics than a conventional 4.5V level spinel-typepositive electrode active material.

When Li₄Ti₅O₁₂ (Li[Li_(1/3)Ti_(5/3)]O₄) is used in the negativeelectrode, a 3V level battery can be obtained.

Japanese Laid-Open Patent Publication No. 2001-210324 proposes the useof a titanium-based oxide in the negative electrode. However, thepublication only discloses in the body thereof a positive electrodeactive material which shows a positive electrode capacity in thepotential range of 4.3 to 3.5 V. This is merely conventional LiMn₂O₄ ora positive electrode active material obtained by adding a trace amountof element to LiMn₂O₄ with the aim of improving the cycle life etc.,which differs clearly from the positive electrode active material of thepresent invention with a charge/discharge range of 4.7 V. Therefore, thebattery system disclosed in Japanese Laid-Open Patent Publication No.2001-210324 is a 2.5V level battery system.

The battery system in accordance with the present invention, on theother hand, has a practical charge/discharge range of 2.5 to 3.5 V, thesame range as currently available 3V level lithium primary batteries. Inaddition, the battery system in accordance with the present inventioncan be widely used because only one battery of the present invention issufficient in devices that requires two dry batteries, and thus isadvantageous.

In other words, a battery voltage difference of 0.5 V between thebattery systems appears as an advantage or a disadvantage for practicaluse in the market. The 2.5V level battery system of Japanese Laid-OpenPatent Publication No. 2001-210324 does not practically provide a greatvalue. Further, Japanese Laid-Open Patent Publication No. Hei 9-147867proposes a positive electrode active material with a charge/dischargepotential of not less than 4.5 V. It also discloses a battery system inwhich carbon is used in the negative electrode, and its object is torealize a lithium ion battery with a high voltage of 4.5 V level, whichdiffers from an object of the battery system in accordance with thepresent invention.

In the positive electrode active material in accordance with the presentinvention, a voltage difference in the discharge curve at the end ofdischarging can be freely controlled. This makes it possible to detectthe remaining capacity if a battery system is appropriately selected. Aspreviously stated, batteries that exhibit a flat-shaped discharge curve(discharge voltage) are more advantageous to electronic devices. Thisis, however, a disadvantage from the viewpoint of detection of theremaining capacity. According to the present invention, however, it ispossible to design a positive electrode active material with aflat-shaped discharge curve in which a voltage difference can be freelycontrolled at the end of discharging.

Therefore, it is advantageous to use Li₄Ti₅O₁₂ (Li[Li_(1/3)Ti_(5/3)]O₄)as a negative electrode active material because the negative electrodepreferably has a flat-shaped discharge curve.

The Li₄Ti₅O₁₂ (Li[Li_(1/3)Ti_(5/3)]O₄) and the positive electrode activematerial in accordance with the present invention have nearly the samecapacity density. Accordingly, it is possible to obtain positive andnegative electrode plates having the same thickness by using them inproducing a battery. This is also an advantage in terms of batterycharacteristics. In commercially available battery systems withLiCoO₂/graphite or LiMn₂O₄/graphite, since the negative electrode has ahigh capacity density, a great difference occurs in thickness betweenthe positive and negative electrode plates. This difference leads to adifference in diffusion of an electrolyte solution into electrodes. As aresult, the rate balance between the positive and negative electrodes isdisturbed, and a load is imposed on either of the electrode plates,accelerating the degradation of a battery.

This indicates that it is preferable to produce a battery system bycombining the positive electrode active material in accordance with thepresent invention and Li₄Ti₅O₁₂ (Li[Li_(1/3)Ti_(5/3)]O₄).

The above-mentioned negative electrode active material exhibits a flatcharge/discharge curve of 1.55 V relative to lithium. FIG. 17 shows thecharge/discharge behavior of the battery system in whichLi[Ni_(1/2)Mn_(3/2)]O₄ is used in the positive electrode andLi[Li_(1/3)Ti_(5/3]O) ₄ is used in the negative electrode. FIG. 18 showsthe cycle life of the battery system until 200 cycles. In FIG. 17, thehorizontal axis represents discharge capacity of the positive electrodeactive material per unit weight. The charge/discharge was performedunder the conditions of a current density of 0.17 mA/cm² and a constantcurrent charge discharge of between 0 to 3.5 V.

As is apparent from FIG. 17, the battery system in accordance with thepresent invention exhibits a flat charge/discharge voltage with anaverage voltage of about 3.2 V as well as a voltage difference at theend of discharging. By using this voltage difference, it is possible torealize the display function that displays an accurate remainingcapacity or the power off alarm function. This battery system has ausable charge/discharge range of 2.5 to 3.5 V, which is the same as thatof 3V level lithium primary batteries.

FIG. 19 shows the rate capability of this battery system with load. InFIG. 19, (a) to (f) respectively represent discharge behaviors at acurrent density of 0.1 mA/cm², 0.17 mA/cm², 0.33 mA/cm², 0.67 mA/cm²,1.0 mA/cm² and 1.67 mA/cm². Also from FIG. 19, it can be observed thatthe difference clearly appears in the discharge voltage although theload is significantly changed.

Contrary to the above, it is also possible to prevent this differencefrom appearing. FIG. 20 shows an example thereof. It is apparent thatany clear difference did not appear even after the load was increased.The positive electrode active material used here was prepared by thefirst baking at 1000° C. and the second baking (reoxidation) at 700° C..Additionally, (a) to (e) in FIG. 20 respectively represent dischargebehaviors at a current density of 0.17 mA/cm², 0.33 mA/cm², 1.0 MA/cm²,1.67 mA/cm² and 3.33 mA/cm².

The above-described negative electrode active material is a zero-straininsertion material that does not expand or contract duringcharging/discharging whereas graphite greatly expands and contractsduring charging/discharging. The positive electrode active material inaccordance with the present invention also does not greatly expand orcontract during charging/discharging. By using this combination, it ispossible to design a battery system in which the expansion andcontraction does not practically occur. Accordingly, the degradation incycle life, rate characteristics and temperature characteristicsresulting from the degradation of active materials and the leakage ofelectrolyte solution outside the battery system due to the expansion andcontraction is significantly improved.

FIG. 21 shows the expansion and contraction during charging/dischargingmeasured by a dilatometer. The positive and negative electrode platesrespectively had a thickness of 60 μm and 110 μm, and the thicknesschange of one stack obtained from the combination of one positiveelectrode plate and one negative electrode plate was measured.

FIG. 21 indicates that the measurement was performed with high precisionbecause the expansion and contraction appear in response tocharging/discharging. The difference thereof is about 1 μm, which onlyaccounts for 0.6% of the battery. Since it is well known thatLi[Li_(1/3)Ti_(5/3)]O₄ of the negative electrode is a material with nodistortion that does not expand or contract at all, even if the changein the negative electrode is not taken into consideration, it can besaid that only a 2% change occurred in the thickness of the positiveelectrode active material. When a conventional LiCoO₂/graphite typebattery is charged, the thickness of the positive electrode expands byabout 5% and that of the negative electrode expands by about 20%;therefore, the degree of the expansion and contraction isextraordinarily small in the battery in accordance with the presentinvention. Such extremely small expansion and contraction duringcharging/discharging is the main factor for longer cycle life. Accordingto the present invention, the cycle life particularly when the batteryis charged and discharged at a high rate is significantly improved,compared to conventional battery systems.

(7) Battery Capacity Design

When the capacity load of the battery is designed, it is necessary toregulate negative or positive electrode-limited capacity of either thepositive or negative electrode. The capacity load is intentionallydesigned according to its application to the devices to be used, thecharacteristics of the materials to be used or the like. In the 3V levelbattery system in accordance with the present invention, the capacity ofthe negative electrode is preferably regulated. Specifically, the ratio(weight) of the negative electrode active material to the positiveelectrode active material should be set at not less than 0.5 and notgreater than 1.2. When the ratio is 1.2, it appears that the positiveelectrode active material is formally regulated. However, since thetheoretical charge/discharge capacity of the negative electrode activematerial per gram exceeds that of the positive electrode active materialper gram, the negative electrode active material is practicallyregulated.

The following describes the reason why the battery system with regulatednegative electrode is more preferred. A positive electrode usually has apotential of about 4.7 V, but, depending on the electrolyte solution tobe used, it may have poor oxidation resistance. Accordingly, in terms ofstability of electrolyte solution, it is disadvantageous to perform thecompletion of charging by increasing the potential of a positiveelectrode. Additionally, it is conceivable that, when lithium elementsare completely removed from the positive electrode active material,oxygen is gradually released to cause the degradation of the activematerials or the oxidation of the electrolyte solution due to oxygen,leading to the degradation of cycle life and battery characteristics.

(8) Current Collector for Positive and Negative Electrode Plates

Lithium ion secondary batteries currently available typically use apositive electrode current collector made of aluminum and a negativeelectrode current collector made of copper. They are used from theviewpoint of the potential of each electrode, and because they aresuperior in corrosion resistance. Japanese Laid-Open Patent PublicationsNo. Hei 9-147867 and No. 2001-210324 explicitly states the use ofaluminum and copper as positive and negative electrode currentcollectors, respectively.

The non-aqueous electrolyte secondary battery in accordance with thepresent invention preferably uses aluminum or an aluminum alloy in bothpositive and negative electrodes. The reason is as follows.

First, it is possible to reduce the battery weight as well as the costby using aluminum instead of copper. In commercially available batterysystems with a negative electrode made of graphite, because thepotential of graphite is as small as 0.2 V or less relative to that oflithium metal, it was impossible to use aluminum in a current collector.This is because aluminum starts reacting with lithium ions at a higherpotential than a potential at which the graphite in the negativeelectrode charges and discharges. In the battery system in accordancewith the present invention, however, the charge/discharge potential ofthe negative electrode is as high as 1.5 V. This means aluminum, whichdoes not start reacting unless the potential reaches that value or less,can be used. Further, when copper is used and the potential of thenegative electrode is increased due to deep discharge or the like,copper ions are leached into the electrolyte solution. The copper ionsare deposited on the negative electrode by recharging before the lithiuminsertion reaction, which inhibits the lithium insertion reaction. As aresult, lithium is deposited as metal on the surface of the negativeelectrode in the form of needle-like crystals. This causes thedeterioration of safety of the battery and the degradation of cyclelife. The use of aluminum, however, does not cause the leach of metalions or the redeposition.

When the charger for battery systems having a negative electrode withregulated capacity is out of order, excessive amount of lithium issupplied to the negative electrode by overcharging. In this case, if thenegative electrode has a current collector made of copper, excessiveamount of lithium is deposited on the negative electrode. Suchneedle-like crystals of lithium metal deteriorate the safety againstovercharging of the battery. Aluminum, however, has sufficientcapability of absorbing lithium. Therefore, when aluminum is used for acurrent collector for the negative electrode, lithium metal can beabsorbed into the current collector without allowing the lithium metalto be deposited on the negative electrode during overcharging. As aresult, the safety against overcharging of the battery will not bedegraded.

(9) Non-Aqueous Electrolyte Solution

The preferred electrolyte solution of the 3V level non-aqueouselectrolyte secondary battery in accordance with the present inventionis described. An organic solvent to be used as an electrolyte solutionpossesses a potential window. Potential window is a gauge for oxidationresistance and reduction property, and it can be said that the wider thepotential window is, the more stable the organic solvent is. In atypical LiCoO₂/graphite type non-aqueous electrolyte secondary battery,oxidation resistance is required until around 4.5 V which is thecharge/discharge potential of cobalt and reduction resistance isrequired until around 0 V which is the charge/discharge potential ofgraphite (hereinafter, a potential relative to lithium metal is referredto as “potential”). Accordingly, the use of an organic solvent without apotential window that satisfies these requirements has been avoided.

Particularly, in the case of using graphite in the negative electrode toimprove the reduction resistance, the use of a lactone type organicsolvent has been regarded as difficult. Likewise, the use of propylenecarbonate has been regarded as difficult because propylene carbonate isalso decomposed during the charging/discharging of graphite. Thesesolvents are less expensive, have a high dielectric constant andtherefore are capable of completely dissolving an electrolyte solution(salt) and superior in oxidation resistance. However, the use thereof isdifficult. For the same reason, the use of trimethyl phosphate andtriethyl phosphate is difficult although they are effective inextinguishing fire and superior in safety.

In the battery system in accordance with the present invention, all theabove-mentioned solvents with useful characteristics can be used. Sincethe non-aqueous electrolyte secondary battery in accordance with thepresent invention use Li₄Ti₅O₁₂ (Li[Li_(1/3)Ti_(5/3)]O₄) in the negativeelectrode instead of graphite, the potential of the negative electrodeis increased up to 1.5 V. Therefore, the reduction resistance which thesolvent is required to have is significantly reduced. Due to thecharge/discharge typical of graphite, the solvent such as propylenecarbonate that is usually decomposed on the surface of the negativeelectrode can be used as an effective solvent.

Although the potential of the positive electrode is increased up to 4.7V or more, these solvents can be used without any problem because theoxidation resistance thereof is not less than 5 V. It is considered thatsolvents superior in oxidation resistance such as sulfolane and methyldiglyme are suitable for the battery system of the present invention. Itis also possible to use conventional solvents such as DEC (diethylcarbonate), MEC (methyl ethyl carbonate) and DMC (dimethyl carbonate) asa diluent for a solvent with high viscosity.

The electrolyte solution that can be used in the present invention isnot specifically limited, and conventional ones such as LiPF₆, LiBF₄ anda lithium salt of an organic anion can be used. In a conventionalLiCoO₂/graphite type non-aqueous electrolyte secondary battery, asolvent mixture prepared by diluting EC (ethylene carbonate) having ahigh dielectric constant and extremely high viscosity with a solventwith low viscosity has been widely used in order to use graphite or todissolve an electrolyte. For the reasons provided above, in the batterysystem in accordance with the present invention, it is possible toselect the most suitable electrolyte solution according to thecharacteristics desired in the devices to be used without anylimitation.

(10) Separator

A typical LiCoO₂/graphite type battery usually uses a porous film madeof polyethylene or propylene as the separator. The separator is fairlyexpensive because the thin porous film is produced by melt-extruding apolymer material, which is then rolled in two axial directions. The mainreason for requiring this film is considered as follows.

The potential of graphite used in the negative electrode is reduced toabout the potential at which the lithium metal is deposited. Thiscreates various defects. In some cases, a trace amount of lithium ispartly deposited on the graphite surface by rapid charging or chargingat a low temperature, and in some cases, cobalt or metal impurities areleached out by an excessive floating charge and deposited on thenegative electrode.

In such cases, in the above-mentioned porous film having micropores,needle-like metal deposits can be suppressed by physical force, whereas,in a separator with larger micropores such as non-woven fabric, a microshort-circuit occurs in a short period of time. Further, separators havea shutdown function to suppress the increase of battery temperature atthe time of overcharging in order to secure the safety againstovercharging in the case where a charger is out of order. The functionis to stop the current between electrodes by crushing the micropores ofa separator when the temperature reaches a certain temperature (about135° C.). For the above reason, an expensive porous film has been usedin a conventional LiCoO₂/graphite type battery.

The negative electrode of the battery system in accordance with thepresent invention, on the other hand, has a potential of 1.5 V, which ismuch different from the potential at which lithium is deposited.Accordingly, the problem described above does not occur. Since lithiumis absorbed when aluminum is used as a current collector for thenegative electrode, such problem as metal deposition does not arise atall. Additionally, the positive electrode active material of the presentinvention does not contain excessive amount of lithium element like acobalt type positive electrode active material, and therefore thebattery system with the positive electrode active material of thepresent invention is extremely superior. In other words, it is notrequired to have the shutdown function with high precision that porousfilms have. For these reasons, in the battery system of the presentinvention, it is possible to use a non-woven fabric by preferably usinga current collector for the negative electrode made of aluminum or analuminum alloy.

Since a non-woven fabric is capable of retaining a large amount ofelectrolyte, rate characteristics, particularly pulse characteristics,can be greatly improved. In addition, unlike porous films, an advancedand complicated process is not necessary, so that a vast choice ofseparator materials can be obtained and the cost can be made lower.Considering its application to the battery system of the presentinvention, it is preferable to use, for example, polyethylene,polypropylene, polybutylene terephthalate and mixtures thereof asmaterials for the separator. Polyethylene and polypropylene are stableto electrolyte. In the case where the strength is required at hightemperatures, polybutylene terphthalate is preferred. The preferredfiber diameter is about 1 to 3 μm. The non-woven fabric whose fibers arepartly joined by a calendar-rolling technique is effective in reducingthe thickness or increasing the strength.

(11) Non-Aqueous Electrolyte Secondary Battery

The following describes other constituent materials that can be used inproducing a non-aqueous electrolyte solution (lithium) secondary batteryincluding the positive electrode active material of the presentinvention.

As a conductive material in the positive electrode active materialmixture used for producing the positive electrode in the presentinvention, any electron conductive material can be used without anylimitation as far as it does not cause any chemical change in theproduced battery. Examples thereof include graphites such as naturalgraphite (flake graphite and the like) and artificial graphite; carbonblacks such as acetylene black, ketjen black, channel black, furnaceblack, lamp black and thermal black; electroconductive fibers such ascarbon fiber and metal fiber; carbon fluoride; powdered metals such ascopper, nickel, aluminum and silver; electroconductive whiskers such aszinc oxide whisker and potassium titanate whisker; electroconductivemetal oxides such as titanium oxide; and conductive organic materialssuch as polyphenylene derivative. They may be used singly or in anyarbitrary combination thereof within the scope that does not impair theeffect of the present invention.

Among them, particularly preferred are artificial graphite, acetyleneblack and powdered nickel. The amount of the conductive material is notspecifically limited, but preferred amount is 1 to 50 wt %, and morepreferred is 1 to 30 wt %. In the case where carbon or graphite is used,the preferred amount is 2 to 15 wt %.

The preferred binder to be used in the positive electrode materialmixture of the present invention is a polymer with a decompositiontemperature of 300° C. or more. Examples thereof include polyethylene,polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride(PVDF), tetrafluoroethylene-hexafluoroethylene copolymer,tetrafluoroethylene-hexafluoropropylene copolymer (FEP),tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), vinylidenefluoride-hexafluoropropylene copolymer, vinylidenefluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylenecopolymer (ETFE resin), polychlorotrifluoroethylene (PCTFE), vinylidenefluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylenecopolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE),vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymerand vinylidene fluoride-perfluoromethylvinylether-tetrafluoroethylenecopolymer. They may be used singly or in any arbitrary combinationthereof within the scope that does not impair the effect of the presentinvention.

Among them, particularly preferred are polyvinylidene fluoride (PVDF)and polytetrafluoroethylene (PTFE).

As a current collector for the positive electrode, any electronicconductor can be used without any limitation as far as it does not causeany chemical change in the produced battery. The materials used for thecurrent collector include, for example, stainless steel, nickel,aluminum, titanium, various alloys, various carbons, and complexescomprising aluminum or stainless steel with the surface treated withcarbon, nickel, titanium or silver.

Particularly, aluminum or an aluminum alloy is preferred. The surface ofthese materials may be oxidized. The surface of the current collectormay be roughened to have concave and convex shape by surface treatment.The form thereof may be any form that is used in the field of batteries.There are, for example, a foil, a film, a sheet, a net, a punched sheet,a lath, a porous sheet, a foam, a molded article formed by fiber bundleand non-woven fabric. The thickness is not specifically limited, butpreferably 1 to 500 μm.

As a negative electrode active material (negative electrode material)that can be used in the present invention, a titanium oxide such asLi₄Ti₅O₁₂(Li[Li_(1/3)Ti_(5/3)]O₄) is particularly preferred. By usingthis negative electrode, it is possible to obtain a 3V level battery, tosolve conventional problems and to greatly improve the batteryperformance as described above. On the other hand, it is also possibleto use the positive electrode active material in accordance with thepresent invention alone. In this case, the following negative electrodecan be used.

As a material for the negative electrode, any material capable ofabsorbing and desorbing lithium ions can be used such as lithium, alithium alloy, an alloy, an intermetallic compound, a carbonaceousmaterial, an organic compound, inorganic compound, a metal complex andan organic polymer compound. They may be used singly or in any arbitrarycombination threreof within the scope that does not impair the effect ofthe present invention.

Examples of the lithium alloy include a Li—Al based alloy, a Li—Al—Mnbased alloy, a Li—Al—Mg based alloy, a Li—Al—Sn based alloy, a Li—Al—Inbased alloy, a Li—Al—Cd based alloy, a Li—Al—Te based alloy, a Li—Gabased alloy, a Li—Cd based alloy, a Li—In based alloy, a Li—Pb basedalloy, a Li—Bi based alloy and Li—Mg based alloy. In this case, theamount of lithium is preferably not less than 10 wt %.

Examples of the alloy and the intermetallic compound include a compoundcomprising a transition metal and silicon, a compound comprising atransition metal and tin, etc. Particularly, a compound comprisingnickel and silicon is preferred.

Examples of the carbonaceous material include coke, pyrolytic carbon,natural graphite, artificial graphite, mesocarbon microbeads, graphitemesophase particles, gas phase growth carbon, virtified carbons, carbonfiber (polyacrylonitrile fiber, pitch fiber, cellulous fiber, gas phasegrown carbon fiber), amorphous carbon and carbons obtained by bakingorganic materials. They may be used singly or in any arbitrarycombination thereof within the scope that does not impair the effect ofthe present invention. Among them, preferred are graphite materials suchas graphite mesophase particles, natural graphite and artificialgraphite.

The carbonaceous material may contain, besides carbon, a differentcompound such as O, B, P, N, S, SiC and B₄C. The amount thereof ispreferably 0 to 10 wt %.

As the inorganic compound, there are, for example, a tin compound and asilicon compound. As the inorganic oxide, there are, other than atitanium oxide mentioned above, a tungsten oxide, a molybdenum oxide, aniobium oxide, a vanadium oxide, an iron oxide, etc.

As the inorganic chalccogenide, there can be used, for example, ironsulfide, molybdenum sulfide and titanium sulfide.

As the organic polymer compound, there are, for example, polythiopheneand polyacethylene. As the nitride, there are, for example, cobaltnitride, copper nitride, nickel nitride, iron nitride, manganesenitride, etc.

These negative electrode materials may be combined such as carbon and analloy, or carbon and an inorganic compound.

The carbonaceous material used in the present invention preferably has amean particle size of 0.1 to 60 μm, more preferably 0.5 to 30 μm. Thepreferred specific surface area is 1 to 10 m²/g. Further, graphite witha distance between carbon hexagonal planes (d002) of 3.35 to 3.40 Å anda size of crystallites in c-axis direction (LC) of not less than 100 Åin the crystal structure there of is preferred.

In the present invention, a negative electrode material containing no Li(carbon or the like) can be used since the positive electrode activematerial contains Li. Such negative electrode material containing no Limay contain a trace amount of Li (about 0.01 to 10 parts by weight of100 parts by weight of negative electrode material) because even if partof Li becomes inactive as a result of its reaction with the electrolyte,Li can be supplied from the negative electrode material.

In order to incorporate Li into the negative electrode active materialas described above, for example, heated or melted lithium metal may beapplied on the current collector with the negative electrode materialattached thereon to impregnate the negative electrode material with Li,or Li may be electrochemically doped to the negative electrode materialin the electrolyte solution by previously adding (pressure welding orthe like) lithium metal to the electrode group.

As the conductive material in the negative electrode material mixture,similar to the conductive material in the positive electrode materialmixture, any electronic conductive material can be used without anylimitation as far as it does not cause any chemical change in theproduced battery. In the case of using a carbonaceous material in thenegative electrode material, the negative electrode material mixture maynot contain the conductive material because the carbonaceous materialitself has electron conductivity.

The binder used in the negative electrode material mixture may be athermoplastic resin or a thermosetting resin, and preferred is a polymerwith a decomposition temperature of 300° C. or higher. Examples includepolyethylene, polypropylene, polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVDF), styrene-butadiene rubber,tetrafluoroethylene-hexafluoropropylene copolymer (FEP),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA),vinylidene fluoride-hexafluoropropylene copolymer, vinylidenefluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylenecopolymer (ETFE resin), polychlorotrifluoroethylene (PCTFE), vinylidenefluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylenecopolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE),vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymerand vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylenecopolymer. Styrene-butadiene rubber, polyvinylidene fluoride,styrene-butadiene rubber and the like can also be used.

When a titanium oxide such as Li₄Ti₅₀₁₂ (Li[Li_(1/3)Ti_(5/3)]O₄) is usedas the negative electrode active material, the current collector for thenegative electrode is preferably made of aluminum or an aluminum alloyfor the previously-mentioned reason.

When a negative electrode active material other than the above is used,the following can be used. Any electronic conductor can be used withoutany limitation as far as it does not cause any chemical change in theproduced battery. As materials constituting the current collector, thereare used stainless steel; nickel; copper; titanium; carbon; copper orstainless steel with the surface treated with carbon, nickel, titanium,or silver; and an Al—Cd alloy, for example. Particularly, copper or acopper alloy is preferred. The surface of these materials may beoxidized. The surface of the current collector may be roughened bysurface treatment. As the form of the negative electrode currentcollector, similar to the case of the positive electrode, there areused, for example, a foil, a film, a sheet, a net, a punched sheet, alath, a porous sheet, a foam and a molded article formed by fiberbundle. The thickness is not specifically limited, but the one with athickness of 1 to 500 μm is preferably used.

The electrode material mixture may contain a filler, a dispersing agent,an ion conducting material, a pressure reinforcing agent and othervarious additives, other than the conductive material and the binder.The filler may be any fibrous material as long as it does not cause anychemical change. Typically used are an olefin polymer fiber such aspolypropylene and polyethylene, a glass fiber and a carbon fiber. Theamount of the filler is not specifically limited, but preferred is 0 to30 wt %.

The positive and negative electrodes used in the present invention mayhave, in addition to the material mixture layer containing the positiveelectrode active material or the negative electrode material, a basecoat layer for enhancing the adhesion between the current collector andthe material mixture layer, the conductivity, the cycle characteristicsand the charge/discharge efficiency, and a protective layer formechanically and chemically protecting the material mixture layer. Thebase coat layer and the protective layer may contain a binder,conductive particles or non-conductive particles.

As the separator, a non-woven fabric is particularly preferred aspreviously mentioned when a titanium oxide such asLi₄Ti₅O₁₂(Li[Li_(1/3)Ti_(5/3)]O₄) is used in the negative electrodeactive material. When a negative electrode active material other thanthe above is used, the following can be used. An insulating microporousthin film with high ion permeability and a certain mechanical strengthcan be used. The film preferably has the function of closing the poresand increasing the resistance at a temperature of 80° C. or higher. Fromthe viewpoint of the resistance to an organic solvent andhydrophobicity, there is used a sheet or a non-woven fabric made ofpolypropylene, polyethylene, an olefin polymer prepared by combining theabove, or glass fiber.

The separator preferably has a pore size that does not allow the activematerial, the binder and the conductive material separated from theelectrode sheet to pass, preferably 0.1 to 1 μm. The thickness of theseparator is usually 10 to 300 μm. The porosity is determined inaccordance with the permeability of electrons or ions, the materials tobe used and the film thickness; it is preferably 30 to 80%. The use of aflame retardant or incombustible material such as glass or a metal oxidefilm further improves safety of the battery.

As the non-aqueous electrolyte solution that can be used in the presentinvention, the electrolyte solution previously described is particularlypreferred when a titanium oxide such asLi₄Ti₅O₁₂(Li[Li_(1/3)Ti_(5/3)]O₄) is used in the negative electrodeactive material. When a negative electrode active material other thanthe above is used, the following electrolyte solution can be used.

The electrolyte solution is made of a solvent and a lithium saltdissolved in the solvent. The preferred solvent is single ester or anester mixture. Particularly preferred are cyclic carbonates, cycliccarboxylic acid esters, non-cyclic carbonates and aliphatic carboxylicacid esters. More preferred are solvent mixtures containing cycliccarbonates and non-cyclic carbonates, solvent mixtures containing cycliccarboxylic acid esters, solvent mixtures containing cyclic carboxylicacid esters and cyclic carbonates.

Examples of the above-mentioned solvent and other solvent that can beused in the present invention are given below.

As the ester to be used as the non-aqueous solvent, there are, forexample, cyclic carbonates such as ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate (BC) and vinylene carbonate (VC),non-cyclic carbonates such as dimethyl carbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC), and dipropyl carbonate(DPC), aliphatic carboxylic acid esters such as methyl formate (MF),methyl acetate (MA), methyl propionate (MP) and ethyl propionate (MA),and cyclic carboxylic acid esters such as γ-butyrolactone (GBL).

As the cyclic carbonate, EC, PC, VC and the like are particularlypreferred. As the cyclic carboxylic acid ester, GBL and the like areparticularly preferred. As the non-cyclic carbonate, DMC, DEC, EMC andthe like are preferred. Optionally, aliphatic carboxylic acid esters mayalso be used. The amount of the aliphatic carboxylic acid ester ispreferably 30% or less of the total weight of the solvent, and morepreferably 20% or less.

The solvent of the electrolyte solution of the present invention maycontain a well-known aprotic organic solvent, in addition to the aboveester in an amount of 80% or more.

As the lithium salt dissolved in the solvent, for example, there areLiClO₄, LiBF₄, LiPF₆, LiAlCl₄, LiSbF₆, LiSCN, LiCF₃SO₃, LiCF₃CO₂,Li(CF₃SO₂)₂, LiAsF₆, LiN(CF₃SO₂)₂, LiB₁₀Cl₁₀, lithium lower aliphaticcarboxylate, chloroborane lithium, lithium tetraphenyl borate, andimides such as LiN(CF₃SO₂)(C₂F₅SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂ andLiN(CF₃SO₂)(C₄F₉SO₂). These salts can be used in the electrolytesolution alone or in any combination thereof within the scope that doesnot impair the effect of the present invention. Among them, it isparticularly preferable to add LiPF₆.

For the non-aqueous electrolyte solution used in the present invention,an electrolyte solution containing at least ethylene carbonate and ethylmethyl carbonate, and LiPF₆ as a lithium salt, is particularlypreferable. An electrolyte solution containing GBL as the main solventis also preferred, and in this case, it is preferable to add an additivesuch as VC in an amount of several %, and to use a salt mixture of LiBF₄and LiN(C₂F₅SO₂)₂ as the lithium salt instead of LiPF₆.

The amount of the electrolyte solution used in the battery is notparticularly specified, but a suitable amount should be used accordingto the amounts of the positive electrode active material and thenegative electrode material and the size of the battery. The amount ofthe lithium salt to be dissolved in the non-aqueous solvent is notparticularly specified, but preferred amount is 0.2 to 2 mol/l, and morepreferably from 0.5 to 1.5 mol/l.

This electrolyte solution is usually impregnated or filled into theseparator comprising, for example, a porous polymer, glass filter, ornon-woven fabric before use. In order to make the electrolyte solutionnonflammable, a halogen-containing solvent such as carbon tetrachlorideor chlorotrifluoroethylene may be added into the electrolyte solution.Also, a carbon dioxide gas may be added into the electrolyte solution inorder to confer suitability for high temperature storage.

Instead of the liquid electrolyte, the following solid electrolyte canalso be used. The solid electrolyte is classified into inorganic andorganic solid electrolytes.

As the inorganic solid electrolyte, nitrides of Li, halides of Li, andoxysalt of Li are well known. Among them, Li₄SiO₄, Li₄SiO₄—LiI—LiOH,xLi₃PO₄-(1-x)Li₄SiO₄, Li₂SiS₃, Li₃PO₄—Li₂S—SiS₂ and phosphorus sulfidecompound are effectively used.

As the organic solid electrolyte, polymer materials such as polyethyleneoxide, polypropylene oxide, polyphosphazone, polyaziridine, polyethylenesulfide, polyvinyl alcohol, polyvinylidene fluoride,polyhexafluoropropylene, and their derivatives, their mixtures and theircomplexes are effectively used.

It is also possible to use a gel electrolyte prepared by impregnatingthe organic solid electrolyte with the above non-aqueous liquidelectrolyte. As the organic solid electrolyte, polymer matrix materialssuch as polyethylene oxide, polypropylene oxide, polyphosphazone,polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidenefluoride, polyhexafluoropropylene, and their derivatives, their mixturesand their complexes, are effectively used. In particular, a copolymer ofvinylidene fluoride and hexafluoropropylene and a mixture ofpolyvinylidene fluoride and polyethylene oxide are preferable.

As for the shape of the battery, any type such as coin type, buttontype, sheet type, cylindrical type, flat type and rectangular type canbe used. In the case of a coin or button type battery, the positiveelectrode active material mixture and negative electrode active materialmixture are compressed into the shape of a pellet for use. The thicknessand diameter of the pellet may be determined according to the size ofthe battery.

In the case of a sheet, cylindrical or rectangular type battery, thematerial mixture containing the positive electrode active material orthe negative electrode material is usually applied (for coating) ontothe current collector, and dried and compressed for use. A well-knownapplying method can be used such as a reverse roll method, direct rollmethod, blade method, knife method, extrusion method, curtain method,gravure method, bar method, casting method, dip method, and squeezemethod. Among them, the blade method, knife method, and extrusion methodare preferred.

The application is conducted preferably at a rate of from 0.1 to 100m/min. By selecting the appropriate applying method according to thesolution properties and drying characteristics of the mixture, anapplied layer with good surface condition can be obtained. Theapplication of the material mixture to the current collector can beconducted on one side of the current collector, or on the both sidesthereof at the same time. The applied layers are preferably formed onboth sides of the current collector, and the applied layer on one sidemay be constructed from a plurality of layers including a mixture layer.The mixture layer contains a binder and an electrically conductivematerial, in addition to the material responsible for the absorbing anddesorbing lithium ions, like the positive electrode active material ornegative electrode material. In addition to the mixture layer, a layercontaining no active material such as a protective layer, a base coatlayer formed on the current collector, and an intermediate layer formedbetween the mixture layers may be provided. It is preferred that theselayers having no active material contain electrically conductiveparticles, insulating particles, a binder and the like.

The application may be performed continuously or intermittently or insuch a manner as to form stripes. The thickness, length, and width ofthe applied layer is determined according to the size of the battery,but the thickness of one face of the applied layer which is dried andcompressed is preferably 1 to 2000 μm.

As the method for drying or dehydrating the pellet and sheet of thematerial mixture, any conventional method can be used. In particular,the preferred methods are heated air, vacuum, infrared radiation, farinfrared radiation, electron beam radiation and low humidity air, andthey can be used alone or in any combination thereof.

The preferred temperature is in the range of 80 to 350° C., and mostpreferably 100 to 250° C.. The water content of the battery as a wholeis preferably 2000 ppm or less, and the water content for the positiveelectrode material mixture, negative electrode material mixture andelectrolyte is preferably 500 ppm or less in view of the cyclecharacteristics.

For the sheet pressing method, any conventional method can be used, buta mold pressing method or a calender pressing method is particularlypreferred. The pressure for use is not particularly specified, but from0.2 to 3 t/cm² is preferable. In the case of the calender pressingmethod, a press speed is preferably from 0.1 to 50 m/min.

The pressing temperature is preferably from room temperature to 200° C..The ratio of the width of the positive electrode sheet to that of thenegative electrode sheet is preferably 0.9 to 1.1, and more preferably0.95 to 1.0.

The content ratio of the negative electrode active material to thepositive electrode material is preferably set to thepreviously-described ratio from the viewpoint of regulating the capacityof the negative electrode when the positive electrode of the presentinvention and a negative electrode made of titanium oxide are used.However, in the case of using the positive electrode active material ofthe present invention alone, although the ratio cannot be specifiedbecause it differs according to the kind of the compound used and theformulation of the mixture, those skilled in the art would set anoptimum value considering the capacity, cycle characteristics andsafety.

The wound electrode structure in the present invention is not requiredto be a true cylindrical shape. It may be in any shape such as anelliptic cylinder whose cross section is an ellipse or a rectangularcolumn having a prismatic shape or a rectangular face.

In the following, the present invention is described usingrepresentative examples, but it is to be understood that the presentinvention is not limited to them.

EXPERIMENT 1

Three different types of positive electrode active material samples wereprepared under the synthesis conditions shown in the above section (3).A mixture obtained by thoroughly mixing [Ni_(1/4)Mn_(3/4)](OH)₂ obtainedthrough a eutectic reaction and LiOH.H₂O was formed into pellets, whichwas then baked to give a positive electrode active material.Accordingly, the composition of the obtained positive electrode activematerial was Li[Ni_(1/2)Mn_(3/2)]O₄. The amount of oxygen was changedaccording to the synthesis condition. The electrochemical analysis ofthe obtained positive electrode active materials was performed in themanner shown in the above section (1).

(i) PRODUCTION EXAMPLE 1

The ambient temperature was increased from room temperature to 1000° C.for about 3 hours, maintained at 1000° C. for 12 hours and thendecreased from 10000° C. to room temperature for 2 hours.

(ii) PRODUCTION EXAMPLE 1

The ambient temperature was increased from room temperature to 10000° C.for about 3 hours, maintained at 1000° C. for 12 hours, decreased from1000° C. to 7000° C. for 30 minutes, maintained at 7000° C. for 48 hoursand then decreased from 7000° C. to room temperature for 1.5 hours.

(iii) PRODUCTION EXAMPLE 1

The ambient temperature was increased from room temperature to 1000° C.for about 3 hours, maintained at 1000° C. for 12 hours and rapidlycooled from 1000° C. to room temperature. Then, the ambient temperaturewas increased to 7000° C. for about 1 hour, maintained at 7000° C. for48 hours and decreased from 7000° C. to room temperature for 1.5 hours.

The electrochemical behaviors of the positive electrode active materialsobtained in Production Examples 1 to 3 are shown as (a) to (c) in FIG.22. Form FIG. 22, it is understood that all of the positive electrodeactive materials show small polarization and a flat charge/dischargecurve. The positive electrode active material (a) of Production Example1 exhibits a voltage difference at the end of discharging, which can beutilized for the detection of the remaining capacity. The difference isas small as only several V, so that the effective detection of remainingcapacity can be achieved without the occurrence of a power-down due tolack of energy when it is used in a device. The positive electrodeactive material (b) obtained through the reoxidation at 700° C. does notshow the difference. This shows that the voltage difference at the endof discharging can be freely controlled in this range by controlling thetemperature and time of the reoxidation process. Similarly, the positiveelectrode active material (c) obtained through the rapid cooling processfirst and then the reoxidation process does not show the difference.This shows that a material with enhanced polarization and flatness canbe obtained by controlling particles as previously stated in the rapidcooling process. Additionally, high density filling can be achieved.

The foregoing showed the case of using the combination of Ni and Mn.Here, the discharge capacity in the case of using transition metalsshown in Table 1 was measured. The baking was performed under the sameconditions as in Production Example 3 described above. The ratio of Mnto other transition metal was the same as 3:1. The discharge capacitiesobtained from each of the positive electrode active materials are shownin Table 1. Table 1 indicates that positive electrode active materialswith similar characteristics were obtained although there weredifferences in capacity.

The result for the case where the ratio of Mn to other transition metalwas 3:1 was the best. When the percentage of a transition metal wasabove or below the above ratio, the capacity at a high potentialdecreased. TABLE 1 Li[Me_(1/2)Mn_(3/2)]O₄ Capacity (mAh/g) Me═Ni 130Me═Cr 128 Me═Co 120 Me═Fe 118 Me═Cu 110

EXPERIMENT 2

A 3V level battery was produced using the positive electrode activematerial in accordance with the present invention in the positiveelectrode and a negative electrode active materialLi₄Ti₅O₁₂(Li[Li_(1/3)Ti_(5/3)]O₄) in the negative electrode. Thenegative and positive electrode plates were produced in the same manner,using the same compound ratio. As the separator, a 25 μm non-wovenfabric made of polybutylene terephthalate was used. The electrode areawas set to 3 cm². As the electrolyte, an organic electrolyte solutionprepared by dissolving 1 mol of LiPF₆ in a solvent mixture of EC and DECin a ratio of 3:7 was used. The positive electrode active material usedhere was the one obtained in the above-mentioned Case 3.

FIG. 23 shows the discharge behavior of this battery system, and FIG. 24shows the high rate characteristics of the same. FIGS. 23 and 24illustrate that the battery system of the present invention is a 3Vlevel battery with excellent polarization characteristics. Further, thepotential shape is unprecedentedly flat.

FIG. 25 shows the pulse discharge characteristics. In FIG. 25, the pulsecharacteristics with the same width can be seen from the start ofdischarging to almost the end of discharging, which is clearly differentfrom the conventional battery which shows gradually increased pulsepolarization at the end of discharging. Accordingly, it is surmised thatthe potential flatness and excellent polarization characteristics likethis have resulted from the optimization of the method for synthesizingthe positive electrode active material and the realization of topotactictwo-phase reactions throughout the discharging.

EXPERIMENT 3

FIG. 26 shows a front view, in vertical cross section, of a cylindricalbattery produced in this example. An electrode assembly 4 obtained byspirally winding positive and negative electrode plates with a separatoris housed in a battery case 1. A positive electrode lead 5 attached tothe positive electrode plate is connected to a sealing plate 2, and anegative electrode lead 6 attached to the negative electrode plate isconnected to the bottom of the battery case 1. The battery case and thelead plate can be formed using a metal or alloy with electronicconductivity and chemical resistance to organic electrolyte. Forexample, a metal such as iron, nickel, titanium, chromium, molybdenum,copper, aluminum, or an alloy made of these metals can be used. Inparticular, the battery case is preferably made of a stainless steelplate or a processed Al—Mn alloy plate, the positive electrode lead ispreferably made of aluminum, and the negative electrode lead ispreferably made of nickel or aluminum. It is also possible to usevarious engineering plastics or the combination of the engineeringplastic and a metal for the battery case in order to reduce the weightof the battery.

Insulating rings 7 are respectively provided on both top and bottom ofthe electrode assembly 4. Subsequently, an electrolyte is chargedthereinto, and the battery case is sealed with the sealing plate. Here,the sealing plate can be provided with a safety valve. Instead of thesafety valve, it may be provided with a conventional safety device. Forinstance, as an overcurrent-preventing device, fuse, bimetal, PTC deviceor the like is used. Besides the safety valve, as a method forpreventing the internal pressure of the battery case from increasing,making a notch in the battery case, cracking the gasket or the sealingplate, or cutting the lead plate can be employed. Alternatively, aprotective circuit including means for preventing overcharge andoverdischarge may be contained in a charger, or may be independentlyconnected to the battery.

As the method for welding the cap, the battery case, the sheet and thelead plate, any well-known method (i.e. AC or DC electric welding, laserwelding or ultrasonic welding) can be used. For the sealing agent forsealing, a conventional compound or mixture such as asphalt can be used.

A positive electrode plate was produced as follows. Ten parts by weightof carbon powder as the electrically conductive material and 5 parts byweight of polyvinylidene fluoride resin as the binder were mixed with 85parts by weight of powdered positive electrode active material of thepresent invention. The resulting mixture was then dispersed intodehydrated N-methylpyrrolidinone to obtain a slurry, which was thenapplied on the positive electrode current collector formed from analuminum foil, followed by drying and pressing, and the foil was cutinto the specified size. A negative electrode plate was produced in thesame manner as the positive electrode plate was produced except thatLi₄Ti₅O₁₂(Li[Li_(1/3)Ti_(5/3)]O₄) was used instead of the positiveelectrode active material.

Styrene-butadiene rubber based binder could also be used. Although atitanium oxide was used as the negative electrode material in thepresent invention, a negative electrode plate could be produced in thefollowing manner when a carbonaceous material was mainly used. Acarbonaceous material and a styrene-butadiene rubber based binder weremixed in a weight ratio of 100:5 to give a mixture and the obtainedmixture was applied onto the both faces of a copper foil, which was thendried, rolled and cut into the specified size to give a negativeelectrode plate.

As the separator, a non-woven fabric or a microporous film made ofpolyethylene was used.

An organic electrolyte solution was prepared by dissolving LiPF₆ in asolvent mixture of ethylene carbonate (EC) and diethyl carbonate in avolume ratio of 3:7 at a concentration of 1.0 mol/l. The obtainedcylindrical battery had a diameter of 14.1 mm and a height of 50.0 mm.

The use of the positive electrode active material in accordance with thepresent invention makes it easier to alarm the remaining capacity. Inview of this, the degree of the voltage difference at the end ofdischarging was adjusted by the reoxidation temperature.

Cylindrical batteries analogous to the above were produced usingLi₄Ti₅O₁₂(Li[Li_(1/3)Ti_(5/3)]O₄) in the negative electrode except thatonly the reoxidation (second baking) temperature was changed in theabove-mentioned Case 3. These batteries were discharged at 1 C rateuntil the battery voltage reached 2.7 V, and the remaining capacity atthat voltage was measured. Subsequently, the remaining capacity whendischarged to 2 V was also measured. Table 2 shows the results. Thevalues are shown in the ratio of the remaining capacity to the wholebattery capacity.

The results of Table 2 indicate that, in the battery system of thepresent invention, the remaining capacity alarm can be easily realizedwithout complicated electronic circuits and calculation. At the sametime, it is possible to freely set the timing of the remaining capacityalarm. TABLE 2 Reoxidation (second baking) Remaining capacitytemperature (° C.) (mAh/g) 700 2.1 800 8.4 900 15.8 1000 17.9

EXPERIMENT 4

The capacity design of the positive and negative electrodes was studied.The cycle life of the cylindrical batteries was measured by changing theamount ratio between the positive and negative electrode activematerials per unit area. The results are shown in Table 3. Regarding thecharge/discharge cycle conditions, the charging was performed at aconstant voltage of 3.5 V and a constant current with the maximumcurrent of 1 C, which was completed 2 hours after charging. Thedischarging was performed at a constant current of 2 C until the voltagereached 2.0 V. Table 3 shows the number of cycles until the remainingcapacity was reduced to 95% of the initial capacity. Table 3 illustratesthat the cycle life decreased when the capacity ratio was 1.2 or more.Accordingly, from the viewpoint of balancing the capacities of thepositive and negative electrodes, it is preferred to substantiallyregulate the capacity of the negative electrode. If the amount of thepositive electrode material is increased more than necessary, thebattery capacity will decrease. Therefore, the capacity ratio ispreferably 0.5 to 1.2. TABLE 3 Negative electrode active Number ofcycles material/Positive electrode until the capacity active materialratio of 95% (cycles) 0.3 280 0.5 302 0.8 305 1.0 299 1.2 290 1.5 260

EXPERIMENT 5

This example examined the current collector for the positive andnegative electrodes. When graphite is used in the negative electrode,the current collector (core member) is usually made of copper because ofpotential and the like.

When Li₄Ti₅O₁₂(Li[Li_(1/3)Ti_(5/3)]O₄) is used in the negative electrodeas previous stated, it is possible to use an aluminum core member.According to the present invention, it was found that this provides theadvantage of improving safety other than the reduction in weight andcost. The reason for this is as follows. When the battery is overchargeddue to the failure of a charger or the like, lithium metal is depositedon the surface of the negative electrode, which causes a lowering ofsafety. When Li₄Ti₅O₁₂ (Li[Li_(1/3)Ti_(5/3)]O₄ is used in the negativeelectrode, the charge/discharge potential is as high as 1.5 V, muchhigher than 0 V at which lithium is deposited. However, when copper isused in the core member, lithium metal may be deposited on the surfaceof the negative electrode. When aluminum is used, on the other hand, thecore member absorbs lithium, inhibiting lithium from being deposited inthe form of lithium metal. Cylindrical batteries produced using each ofthe current collectors shown in Table 4 were put through an overchargingtest, and the highest temperature of the battery surface at that timewas measured. In the overcharging test, the overcharging was performedat a constant current of 1.5 C.

Table 4 indicates that the use of an aluminum core member reduced thebattery heating at the time of overcharging. As describe above, it ispossible to produce a lightweight and highly safe 3V level battery withlow cost by using an aluminum core member in the battery system of thepresent invention. TABLE 4 Material for Battery surface currentcollector temperature(° C.) Copper 45 Aluminum 81

EXPERIMENT 6

In this example, the preferred electrolyte solution for the batterysystem in accordance with the present invention was examined.

Batteries in which graphite is used in the negative electrode have manylimitations regarding the electrolyte. Particularly, the use of alactone type organic solvent has been regarded as difficult from theviewpoint of reduction resistance. Likewise, the use of propylenecarbonate has been regarded as difficult because it is also decomposedduring the charging/discharging of graphite. These solvents areadvantageous because they are inexpensive, have a high dielectricconstant, are capable of completely dissolving an electrolyte salt andare superior in oxidation resistance. For the same reason, the use oftrimethyl phosphate and tryethyl phosphate has been regarded asdifficult. These solvents are effective in extinguishing fire andsuperior in safety. In the present invention, these useful solvents canbe used.

Currently, a conventional electrolyte solution is prepared based onethylene carbonate (EC) with extremely high viscosity due to thenecessity to form a protective film on the surface of graphite and todissolve an electrolyte salt. The present invention does not require theEC. Although the battery system of the present invention can exhibit avoltage as high as 3V, the scope of selection of electrolyte solutioncan be greatly expanded because graphite is not used in the presentinvention. The preferred electrolyte solutions for the battery system ofthe present invention is shown in FIG. 5. In FIG. 5, the capacityobtained by changing the electrolyte is indicated in an index where thecapacity obtained from the conventional electrolyte system is taken as100. For comparison, cylindrical batteries were produced in the samemanner using lithium cobalt oxide in the positive electrode and agraphite material in the negative electrode. The evaluation resultsthereof are also shown in the table.

It is to be noted that, the indication “EC/DEC (3/7)” in the row ofsolvent mixture means a solvent mixture of EC and DEC in a compositionratio of 3:7. The capacity obtained from this electrolyte solution wastaken as 100 in each of the battery systems.

Table 5 illustrates that the present invention can use electrolytesystems that was unable to be used before without any problem andprovide an inexpensive and highly safe battery whereas the conventionalbattery systems using graphite did not exhibit a high capacity at all.Further, it is also possible to use a solvent mixture of these solventsor a combination of conventionally used solvents. TABLE 5 Battery systemLiCoO₂/ Composition of Electrolyte of the present Graphite solventmixture (salt) invention type battery EC/DEC (3/7) 1 M LiPF₆ 100 100 GBL1 M LiPF₄ 102 10 GVL 1 M LiPF₄ 101 12 PC 1 M LiPF₆ 102 2 Methyl diglyme1 M LiPF₆ 100 20 MethoxyEMC 1 M LiPF₆ 100 87 Trimethyl 1 M LiPF₆ 98 18phosphate Triethyl 1 M LiPF₆ 97 20 phosphate Sulfolane 1 M LiPF₆ 87 30PC/DEC 1 M LiPF₆ 100 13 PC/EMC 1 M LiPF₆ 100 12 GBL/PC 1 M LiPF₆ 101 8

EXPERIMENT 7

In this example, the preferred separator for the present invention wasexamined.

The battery system of the present invention does not require a separatorwith high functionality such as porous film. The use of a non-wovenfabric may decrease the safety against overcharging because the shutdownfunction is decreased. However, since a non-woven fabric is capable ofretaining a larger amount of electrolyte than a porous film, theimprovement in pulse characteristics, in particular, can be expected.

Cylindrical batteries in accordance with the present invention wereproduced in the same manner as above using a non-woven fabric made ofdifferent kinds of polymer materials shown in Table 6. Table 6 shows thepulse discharge characteristics and the highest battery surfacetemperature during overcharging. In the pulse discharge, a simple pulsein which a current of 1 A was on for 5 seconds and off for 5 seconds wasperformed. The pulse discharge time of the battery using the separatorshown in Table 6 is indicated in an index where the pulse discharge timeobtained when a conventional PE porous film was used is taken as 100.The overcharging was performed at a constant current of 1.5 C.

Table 6 indicates that, the battery systems in accordance with thepresent invention can greatly improve the pulse discharge time by usingthe non-woven fabric while the safety against overcharging analogous toconventional ones is maintained. It is also apparent that the voltagedecrease due to pulse current can be enhanced by using the non-wovenfabric. TABLE 6 Separator Pulse time Battery surface Material indextemperature (° C.) Polyethylene porous film 100 42 Polyethylenenon-woven fabric 178 45 Polypropylene non-woven fabric 180 46Polybutylene terephthalate 181 50 non-woven fabric

Industrial Applicability

According to the battery system of the present invention, it is possibleto greatly improve the balance of the flatness of discharge voltage, thehigh rate discharge characteristics, the pulse characteristics and thecycle life during the high rate charging/discharging. In the aboveexamples, the secondary battery of the present invention was describedon the premise that it would be used for a portable device. However, thepresent invention is applicable to a power source for electric toolswhich strongly demand the charging/discharging at a high rate and thecycle life during the high rate charging/discharging, and to a largebattery of driving system which can be used as a power source for hybridcars and electric vehicles.

According to the present invention, an inexpensive nickel-manganesecomposite oxide which exhibits flat high voltage can be effectively usedas a positive electrode active matrerial, and by using a titanium oxidein the negative electrode, it is possible to provide a 3V levelnon-aqueous secondary battery with excellent high rate characteristicsand good cycle life.

1. A positive electrode active material represented by the compositionformula: Li_(2±α)[Me]₄O_(8−x), wherein 0≦α<0.4, 0≦x<2, and Me is atransition metal containing Mn and at least one selected from the groupconsisting of Ni, Cr, Fe, Co and Cu, said active material exhibitingtopotactic two-phase reactions during charge and discharge.
 2. Thepositive electrode active material in accordance with claim 1,characterized in that the phase of the transition metal has a 2×2superlattice.
 3. The positive electrode active material in accordancewith claim 1, characterized in that the ratio between Mn and othertransition metal is substantially 3:1.
 4. The positive electrode activematerial in accordance with claim 1, characterized in that said positiveelectrode active material has a spinel-framework-structure and the Liand/or Me exist in the 16(c) site in the space group Fd3m.
 5. Thepositive electrode active material in accordance with claim 1,characterized in that said positive electrode active material hascharge/discharge curves with a potential difference of 0.2 to 0.8 V. 6.The positive electrode active material in accordance with claim 1,characterized in that said positive electrode active material has alattice constant attributed to a cubic crystal of not greater than 8.3Å.
 7. The positive electrode active material in accordance with claim 1,comprising a mixture of crystal particles with a particle size of 0.1 to8 μm and secondary particles of said crystal particles with a particlesize of 2 to 30 μm.
 8. A method for producing a positive electrodeactive material comprising: (1) a step of mixing Mn and a compoundcontaining at least one selected from the group consisting of Ni, Cr,Fe, Co and Cu to give a raw material mixture; or a step of synthesizinga eutectic compound containing a Mn compound and at least one selectedfrom the group consisting of Ni, Cr, Fe, Co and Cu; (2) a step of mixingsaid raw material mixture or eutectic compound with a lithium compound;and (3) a step of subjecting the compound obtained by said step (2) to afirst baking at a temperature of not less than 600° C., whereby apositive electrode active material represented by the formula:Li_(2±α)[Me]₄O_(8−x), where 0≦α<0.4, 0≦x<2, and Me is a transition metalcontaining Mn and at least one selected from the group consisting of Ni,Cr, Fe, Co and Cu, said active material exhibiting topotactic two-phasereactions during charge and discharge is obtained.
 9. The method forproducing a positive electrode active material in accordance with claim8, characterized in that said first baking is performed at a temperatureof not less than 900° C..
 10. The method for producing a positiveelectrode active material in accordance with claim 8, characterized inthat said method further comprises a step of performing a second bakingat a temperature lower than that of said first baking after said firstbaking.
 11. The method for producing a positive electrode activematerial in accordance with claim 10, characterized in that said secondbaking is performed at a temperature of 350 to 950° C..
 12. The methodfor producing a positive electrode active material in accordance withclaim 10, characterized in that said second baking is performed at atemperature of 650 to 850° C..
 13. The method for producing a positiveelectrode active material in accordance with claim 8, characterized inthat said method further comprises a step of rapidly cooling saidpositive electrode active material after said first baking and/or saidsecond baking.
 14. The method for producing a positive electrode activematerial in accordance with claim 13, characterized in that said rapidcooling is performed at a temperature decrease rate of not less than4.5° C./min.
 15. The method for producing a positive electrode activematerial in accordance with claim 13, characterized in that said rapidcooling is performed at a temperature decrease rate of not less than 10°C./min.
 16. The method for producing a positive electrode activematerial in accordance with claim 14, characterized in that said rapidcooling is performed until the temperature reaches room temperature. 17.A non-aqueous electrolyte secondary battery comprising; a positiveelectrode containing the positive electrode active material inaccordance with claim 1; a negative electrode containing a titaniumoxide; and a non-aqueous electrolyte and a separator, characterized inthat said battery has a usable charging/discharging region of 2.5 to 3.5V and a practical average voltage of 3V level.
 18. The non-aqueouselectrolyte secondary battery in accordance with claim 17, characterizedin that said titanium oxide has a spinel structure.
 19. The non-aqueouselectrolyte secondary battery in accordance with claim 17, characterizedin that said titanium oxide is Li₄Ti₅O₁₂.
 20. The non-aqueouselectrolyte secondary battery in accordance with claim 17, characterizedin that said battery has an operating discharge curve with a potentialdifference of 0.2 to 0.8 V.
 21. The non-aqueous electrolyte secondarybattery in accordance with claim 17, characterized in that said positiveand negative electrodes have a current collector made of aluminum or analuminum alloy.
 22. The non-aqueous electrolyte secondary battery inaccordance with claim 17, characterized in that said non-aqueouselectrolyte comprises at least one selected from the group consisting ofpropylene carbonate, γ-butyrolactone, γ-valerolactone, methyl diglyme,sulfolane, trimethyl phosphate triethyl phosphate and methoxymethylethylcarbonate.
 23. The non-aqueous electrolyte secondary battery inaccordance with claim 17, characterized in that said separator is madeof non-woven fabric.
 24. The non-aqueous electrolyte secondary batteryin accordance with claim 23, characterized in that said non-woven fabriccomprises at least one selected from the group consisting ofpolyethylene, polypropylene and polybutylene terephthalate.
 25. Thenon-aqueous electrolyte secondary battery in accordance with claim 17,characterized in that the weight ratio of said negative electrode activematerial to said positive electrode active material is not less than 0.5and not greater than 1.2.